If you like online multiplayer RTS, then get ready for Sins of a Solar Empire 1.5 -- the free-standing Rebellion expansion. If you missed this excellent 4x-RTS, Rebellion is your chance to purchase the entire game--the original Sins, the Entrenchment expansion, the Diplomacy expansion, and the upcoming large Rebellion expansion in one single $40 package.
Sins is a unique, deep RTS that focuses on strategy and tactics while doing a great job of keeping micromanagement to a minimum, preventing it from becoming an APM-based clickfest. It's excellent for online multiplayer PvP. Ironclad and Stardock may not have the advertising budget or cachet of Blizzard, but most Starcraft fans would probably love this game.