Gear: 
Clockwise from top left: Headsets for radios - Flashlights (The Gun is actually a fake metal gun from an arcade game and is NOT functional except as a deterrent, flashlight, and "Attitude Adjuster" It's ONLY carried in the basements of Avalon where we have a known threat of angry homeless people The Geek Group DOES NOT recommend you carry a real sidearm on ANY adventure, unless you're invading France, then it's ok) Waterproof light - Hard tool box (stays in van) - Jump Box (stays in van) - First Aid kit (3 person) - Goggles - Dogtag Canisters (a sealed canister with your name, age, address, contact#, etc) - GOD Light - a few radios - and the "Key" (Bolt cutters), on the bottom is the Whaletail. This is 90% of all the required gear for one *very* serious Urban Explorer. To the side is Chris's backpack.
The 6-D cell MAG-LITE, as big as they come.
The Geek Group's U.E.T. maintains the following gear for our various adventures. You don't use everything for every adventure (tunneling requires a vastly different set of gear than exploring a hospital downtown), but this will help you know what to shop for and what to avoid.
Almost all text below was copied directly from the web site, though they did rearrange it a bit.
Flashlights (flashlights):
Nothing compares to the Mag brand of flashlight. There are a LOT of cheap knock offs out there, watch out. There is a BIG difference between a $8 3-D Cell generic light and a $24 3-D Cell Maglite. You use your light more than anything else and literally trust your life to it on a regular basis, don't skimp and buy crap tools.
The only problem with Maglites is that they store the spare bulb in the tail piece (look under the spring). A truck can run over your light with no problem, but if you drop it 10' or more the filament will break in the bulb. If the spare is in the tail piece, they're both toast. We carry a spare bulb in the tail piece (for convenience and because *most* of the time if the bulb burns out it's from use, not dropping). We also carry extra bulbs in a backpack.
For draining, a waterproof and FLOATING flashlight is a big plus. Maglites sink like a rock.
For flashlights we currently have the following in our arsenal:
GOD LIGHT a 1,000,000 candlepower rechargeable searchlight. It's a great light, but heavy, and it only works for about 10 minutes. Use it only when needed.
5-D Cell Maglite (red, they didn't have black)
3-D Cell Maglite (Black) We have several of these, at least one for each person.
2-AA Cell Mini-MAG (Blue, Red, and Black) Everyone carries these in a holster almost constantly both on and off adventure. You never know when opportunity may present itself.
1-AAA Mini-MAG Solitaire (black) This is a great light for emergency use (if you lost your main and backup, it happens). It's also handy for a LOT of other things, computer work, etc... It's small enough to fit in a key pocket in a pair of jeans, or even tucked into a sock.
Radios:
The new breed of "Personal Radios" have replaced CBs for a lot of our close work. We each carry a 14 channel (with a couple hundred sub-channels) short range personal radio. They are small, lightweight, and will run 15 hours on 4-AAA batteries. They're only good for short range (according to the box, 2 miles...that's measured in the Mojave with nuclear batteries) they're great in a building for team-to-team communications, but not across a campus.
Personal radios are available everywhere from Radio$hack to Meijer, Wal-Mart, etc... for prices ranging from $30 to $130. We would recommend the $60-$80 range. Cheap radios are like cheap flashlights, but $130 is a bit much for most explorers, and if you're gonna spend that much, get a Yaesu or an ICOM 2-meter setup.
Multi Tool:
A GERBER is our top choice, though S.O.G. or Schrade is decent as well. This should be carried in a holster for ease of use. The larger Swiss-Army knives are also very useful. We recommend the Swiss Champ for adventure use. A Multi-tool is much stronger for heavy work though.
First Aid Kit / Jump Box:
Every team should carry a basic 3-man first aid kit throughout any adventure. These can be bought for about $20 and carried in a small belt pack. All members should be well versed in at least basic first aid and CPR though we recommend people getting at least a First Responder certification. If you're going to be traipsing around in tunnels and other dangerous places, you might as well be useful in an emergency situation.
In the adventure van (Geek-0) we also carry a well stocked "Jump Box" which is a large plastic toolbox filled with what amounts to a heavy-duty first aid kit. It's enough to handle almost anything to maintain a person until EMS arrives.
We recommend everyone also carry any medications they may personally need. Asthma, Diabetes, etc.. (though we carry glucose tabs in the Jump Box) require specific medications that may be needed immediately in an emergency situation.
Safety Goggles:
Basic strap-style shatterproof plastic goggles are a must for abandoned building exploration. You are constantly digging around and looking up at catwalks, etc.. and a bit of rust in your eye can ruin your whole weekend. Get a comfortable pair, we use the Scott brand ski goggles.
Gloves:
Plain old "Jersey" work gloves are fine, $5 will buy half a dozen pair. If you're really serious we recommend climbing/rappelling gloves with the fingers cut off.
Hat:
A basic black stocking cap the fits snugly to the top of your head and can be pulled down over your face (impromptu dust mask) and is great in the winter. For hot climates, a baseball cap worn backwards is good. These are mainly used to keep junk out of your hair (lots of cobwebs and spiders in tunnels).
Sharpie (Marker):
The Sharpie brand Permanent marker is an absolute must in many adventures. In tunneling to mark when you turned (think breadcrumbs) and for minor graffiti to let people know how far you made it and when you were there.
Camera / Recording Gear:
It's rather difficult to be taking notes when you're in a steam tunnel sweating your butt off. We take along 1 or 2 cameras among the group, and sometimes a VHS video camera, though it's a pain to carry on certain adventures. A micro cassette recorder is a mush have for taking notes and is easy to clip to a strap on your backpack, set it to VOX and forget about it.
All text below was copied directly from the web site, word for word.
Backpack or bag
You will need
a bag of sorts to carry your kit in. You should have one that keeps your hands
free. A bag you have to carry in your hand just won't cut it. A backpack or
bum-bag should do the job.
A back-pack
should be comfortable and not too big. A simple day pack will do the job.
Ideally it should have padded straps but let's not be fussy, almost any day pack
will do the job. You don't need anything flash, just a hard wearing pack. Hard
wearing is the operative word here, you will need a pack with reasonably thick
material, thin nylon plastic packs will fall apart after a short while
The pack
should be large enough to carry a flashlight, bottle of water and can as well as
having pockets for your spare flashlight, manhole popper and other stuff you
almost always carry. You can pick up a pack almost anywhere but check out
discount shops because they often sell them.
If you
explore drains with a pocket flashlight a bum bag will do just fine. A small flashlight
will fit into a bum bag just fine along with textas etc. You can pick one up for
as little as $2 in most discount shops.
Popper
Manhole poppers are a very handy tool to have. Poppers are used to lift
manholes. They consist of a small 'T' shaped device made out of 7.5 or 8mm steel
rod. You can make these yourself if you can get hold of oxy welding gear. It is
a simple case of cutting up the metal rod into two 12 cm long bars and one 2.5
cm bar with the 2.5 cm bar on the bottom. The joints should be welded together
strongly. I have found the handle on my popper to be a bit hard on the hand and
recommend a wider handle, possibly a metal / wooden one.
The 2.5 cm bottom length of popper is designed to fit into the lifting holes of GATIC type manholes. You may want to create a second popper with a longer bottom piece to allow easier opening of round grid manholes.
Water Bottle
A small water bottle is useful and just about a must have. Exploring drains can be thirsty work. You don't need a custom job, just a small plastic soft drink bottle will do fine.
Footwear
Most drains have some water in them, even if it's not deep. You can either adopt a 'keep my feet dry' attitude or a 'stuff it' attitude. After a while you will probably just give up trying to keep your shoes clean and dry. So I recommend you use shoes you don't mind getting dirty. Old shoes with good grip are best. Grip is important! You must have good grip or you will slip over.
You might consider buying a pair of cheap shoes from a factory shop, seconds place or from Dimmeys in Richmond. Your shoes will require good grip for climbs and crossing slippery drains. In summer when the weather's warm runners are ok. In winter when the water is cold you may consider gumboots, even though they are uncomfortable. Some gum-boots now come with good toe protection and I suggest if you can afford them then they're the best to get. You can usually pick up gumboots at a second hand shop for a few dollars or you can buy them at most hardware or disposals stores.
Clothing
You will mostly need and use typical everyday clothing. There is little need for any specialised clothing. In summer you should do with shorts and shirt. Shorts are recommended because although most drains don't have much water in them, splashing will normally get you lower half of your leg wet. Typically the shorts should be quick drying and have big pockets. In summer, when it's best to explore drains you can normally just wear shorts, a good pair of shoes and a shirt.
Occasionally some people will explore wearing coveralls or overalls, especially in dirty or cold drains.
And remember drains are dirty! Gather a single set of cloths to wear in a drain and wear only them in the drain. They will get dirty, but it will save you messing up other cloths.