Deus Ex is one of the best PC games in history, but even it isn't without its faults and shortcomings. Shifter addresses these and then some. Unused game content, buggy scripted events, several completely useless items, and certain design choices that can only be described as "annoying", all fixed (or at least "improved") by this little mod. I started Shifter in 2002 to fix just one of the latter -- the fact that weapons equipped with a laser sight won't turn it on automatically -- and the project has expanded exponentially since then. A few new weapons, some additional uses for existing items, new augmentations, and several fixes for existing Deus Ex bugs all exist in Shifter, along with a new multiplayer component. The mod is so popular it's been featured in Game Informer's July 2007 edition of Mod World; how many Deus Ex mods can say that? (only 3, to be specific) Shifter: "Removing the suck from Deus Ex" since 2002.
This is what happens when I'm bored and need a break from coding. [UPDATE: New version(s) after even more boredom]