Exploits (DX)

Free skill points
Before talking to Gary Savage on the rooftop at the MJ12 sub base (the default save name at this location is "New West Coast - Sub Base"), make sure your inventory is full. Upon talking to him, you will receive 500 skill points and he will offer you a augmentation canister, but your inventory is full. Repeatedly talk him for 500 more skill points each time.

Bypassing laser beams
Simply walk close to beam(s), do a quicksave and while saving hold down Up Arrow. When the save is complete, you should be on the other side of the beam(s) without triggering any alarms.

Instant Lockpicking
Find a locked door, then use a lockpick on it. As you start to pick the lock, go into any of the menu or subscreens. After 10–20 seconds (it depends on the lock and your skill level), leave the menu and return to the game. You will have instantly picked the lock using a single pick, even if it required more than one! This trick will also work with multitools on cameras, alarms, etc.

NPCs open locked doors
There's an easy, confortable and mostly useful way to open locked doors by pushing a random NPC on the way to a locked door: he will open it for you after touching it. This can sometimes be the best way to open special doors (e.g. leading Kaplan to the door of UNATCO HQ) or common doors. Ateast NPCs do not react, making it a simple, but sometimes slow way.

More skill points at the start
On the character creation screen untrain "Pistol" and start the game. After the intro your skill level will be back at "Trained", but the skill points you gained will still be there.

Trivia: this bug was fixed in demo version Deus Ex.

Moon Jump
With the Speed Enhancement augmentation active, entering far enough into a sub-menu (e.g. escape menu, settings, game options) while the game is paused will re-apply the effects of the aug, greatly increasing movement speed and jump height, as well as reducing fall damage. This can be repeated as many times as you think you can handle.

Free Items
Make sure your inventory is full before attempting this. Go up to anyone who will sell you items and make a purchase. Look down at your feet. The item you tried to purchase will be down there. This can also be done for items given to you like the upgrading canisters from Jaime. This trick can be done as many times as you like as long as your inventory is full.

Bring items back to the first mission
Start a new game in the middle of a mission, any items which you were carrying will be with you at the beginning.

Unlimited inventory space
Once you run out of inventory space, click and drag away a large item from the inventory then press I to click off the screen. The next item you collect will appear under the large item you dragged off, This can be exploited the best with either the GEP gun, the plasma rifle, or the flamethrower.

Infinite crate items
Take any supply into a loading screen. Whichever item was inside the crate will be duplicated and fall into the new map. This can be done infinitely so long as the crate is not broken.

Keep your items
After you are arrested by Gunther, it will load the next map. Quickly when the map loads hit F1, you will have a short timespan to access your inventory. Drop all your items and pick them back up. You will keep your items, and you do not have to retrieve them.

Grenade climbing
Find a wall and face it in the crouching position. Stick a grenade to it (you can disable it, so it won't get triggered by an NPC). Now jump on the grenade and place another one above it. Jump on it too and remove the previous one. Repeat.

Grenade jump
Jump and throw a grenade (preferably EMP, so you won't kill yourself) while aiming down. If you time it correctly, you'll bounce off your own grenade and jump higher than you would be able to otherwise.

Grenade falling
Similar to the previous exploit, throw a grenade down when falling from a high building. You'll bounce off it without taking falling damage.

Grenade clipping
by standing on the grenade thrown near a vertical surface when it flips you get pushed inside the wall. Most of the time it equals a telefrag and your death, but with thin vertical surfaces like glass or fences you get moved to the second side, along with a bonus of weirdly turning camera.