http://warzone2100.sourceforge.net/
Warzone 2100 is totally free and open source now.
Even all the game content is free, just download, install and play
Its a good RTS(from 1999 I believe), you can design your own units so you have really lots of different stuff to make. You can research for new buildings and other things...after a few hours you still haven't researched everything
You can play campaigns, skirmish(vs cpu) or online(no masterserver, so you need an ip).
So if you like RTS game I highly recommend it