The Rate of Fire mod can turn sniper rifles and pistols into semi-automatic assault rifles. With enough upgrades, just about any weapon can shred through enemies by holding the fire button.
Midway through the game, an I-LAW prototype weapon (which combines the rockets from the GEP Gun and the compact size of an assault shotgun) can be found by visiting the ruins of the MJ 12 base underneath Hell's Kitchen. Not only is the weapon extremely compact and powerful, but ammo is easy to come by, as every MJ 12 Commando killed through the rest of the game drops the special type of ammo (L-Rockets) needed for it. It's easy to mow through enemies for the rest of the game with this weapon.
The Railgun (a new secondary weapon found in Paris and on Walton Simons when you defeat him) is the X-51 Farsight from Perfect Dark transplanted into the world of Deus Ex. Not only does this upgraded Plasma Rifle have a precision scope that allows the player to fire through walls, but its range is so absurd that you can shoot enemies from all the way across game maps. It can also One-Hit Kill anything besides the Men in Black enemies and robots. You can stand in the starting position on any map and take down enemies from a half-kilometer away while they run around in circles unsure of what to do. The only downside is that you'll mow through ammo fairly quickly, but even that becomes trivial when the later stages start offering up more plasma clips.
Sometimes, placing objects in JC's office (and letting them transfer over through the early missions) results in additional weapons spawning in the office. This can include modded weapons that may be more powerful than what the player is currently using.
One side-effect of the "points" system (which was added to prevent the player from killing every NPC on the map) was making it so that killing friendlies resulted in a loss of skill points. However, friendly NPC's can still be turned hostile by hitting them a couple times or shooting them in the torso with enough small-arms fire. This can be used as an efficient way to farm skill points more quickly over the course of the game.
