Quantum is a really neat RTS game that I found while perusing The Linux Game Tome. Don't be put off by the name of the repository: the game runs equally well on Linux, Mac, and Windows, because it's programmed in Java. Don't be put off by the programming language, either: it starts and runs quite quickly.
I recommend using the Quantum webstart link, since you'll not only get the up-to-date version, but libraries that are appropriate for your system, too. (This is practically necessary if you run on a 64-bit Linux machine.)
There's no documentation, so I'm writing my own. Read on for my Quantum strategy guide.