The RTS game Warfar Inc. has lots of add-ons
I have Warfar Inc. installed but I've yet to play it. So I started the game up yesterday and noticed that you have the option of downloading add-on maps. I saw tons of new maps which I thought was kind of cool. I downloaded 20 or more maps which gave me like 40+ missions. My point is this feature adds a new level of replayability to this game. Maybe I'm the last one to find out about this, who knows. But it was a pleasant surprise to see these capabilities.
Helvetica will treat you good if you take time to understand her.