StarCraft and Red Alert are two of my favorite real-time strategy (RTS) games. Though these games are closed-source, they are still playable in Linux through Wine. Recently, I have tried a few free and open-source RTS games and was really impressed with some of them. The best thing is that I can play them instantly on my Linux box without the need for Wine and without having to spend a single dime.
If you are into real-time strategy gaming, I highly recommend you check out these Free and Open-source RTS games: