Velocity Vector
LinuxDevCenter has a good article on the Stratagus open source 2D RTS engine:
http://www.linuxdevcenter.com/pub/a/linux/2004/07/15/stratagus.html
The SourceForge project page for Stratagus is here:
http://stratagus.sourceforge.net/
Now for the bad news: According to the project page, this is GPL'd software, so this won't be useful for many of you. However, those of you willing to deal with the terms of the GPL may find this a good base for real time strategy games.