EVE Online. Obviously it's not the only thing that draws players to the game, but the huge player-run wars are a big part of what outsiders know of the game, and they're certainly engaging to be a part of. With the first patch to the Inferno expansion, CCP Games is making some changes to the war system, chiefly regarding allies and how they can be used in a war.
The biggest change is that all ally contracts are set to a two-week timer with no option for automatic renewal. An ally can be re-signed immediately after the initial contract expires, but hiring a larger number of allies will also sport a cost, with that price getting higher and higher as you hire more allies. Allies will also be locked out of participation in mutual wars to help ensure fairness on both sides. Dedicated warmongers may wish to take a look at the full list of upcoming changes to plot out the next few moves in their private little wars.