Defense of the Ancients Mod.
Warcraft 3: Defense of the Ancients is not a mod in the traditional sense of the word (like Counter-Strike is a mod for Half-Life), but it's rather a very, very in-depth map complete with its very own unique gameplay. In this game, which is a "custom game" on Battle.NET (no credit for winning or losing will be given to your account), you're put in charge of managing only one character, which may be one of a large number of original heroes, each with completely unique special abilities. You can upgrade this hero by killing a large number of enemy players or creep, allowing you to level-up and upgrade your abilities. On top of this, you can also use money won by killing enemies to buy stat-altering items, or combining items to form "ultimate items," which take less room in your character's inventory, and provide some massive stat bonuses.
The overall goal in Defense of the Ancients is to protect your base (the game supports up to ten players, five heroes per team, and allied and enemy creep are spawned all the time to help out), while also trying to destroy the enemy base. The average game lasts about an hour and a half, so it's no short game, but when you and a bunch of friends are all dueling it out, DotA can be immensely fun (and even more addicting). If you've got Warcraft 3 and its expansion, the Frozen Throne, be sure to download version 5.75 of the mod, and take joy in its supreme fun.