If you like RTS, and it is available for Mac, try Homeworld or Homeworld Cataclysm. Homeworld came out in 99, so you can get it very cheap and no RTS in it's "genre" have come out to date that can top it.
If you want to try something different in an FPS, try Half Life, another game that is a few years old but still has a huge following that continues to allow the demand for expansions to be created.