I've been playing this free game League of Legends a bunch lately.
http://www.leagueoflegends.com/http://www.youtube.com/watch?v=7aweJLJqsNoIt's 5v5 or 3v3, and it's sort of a top down Warcraft/Starcraft looking game. But in a regular RTS game you control a ton of units and make buildings, and in League of Legends you just control one champion (and don't make buildings). Each team has a base with a final goal called a Nexus that you have to destroy to win the map.
Also throughout the map there are towers that shoot at you if you go too close (unless you have friendly minions with you, which are like bots). And in each base you have a spawning thing that spawns friendly minions that push through the map at the towers.
So it's all about teaming up and killing enemy players in a 5v5 (similar to TDM in urt sort of) while trying to kill all the enemy towers to reach their base and kill their base.
I'm still not great at it and theres more stuff in it I don't know that well yet, but generally speaking I picked it up pretty quick.