I'm a contributor to the development of the the real-time strategy game MegaGlest (http://megaglest.org). This is a rather old game, as you can quickly see by taking a look at the screen shots available on the website. However, it still has a good game play, it is completely free, it works on Windows, OS X, Linux (and more), you can play online, it doesn't take too many resources, the source code is available, and there are couple people dedicated to improving it steadily (which may also result in better graphics in the future). So please look beyond the graphics and consider to not convert this topic into a flame war.
If you're still reading this ;-) then you could really help improve the game by playing it and giving feedback by posting on the forums (https://forum.megaglest.org) - yes you will need to register but that's a matter of 2 minutes. We just released v3.9.1 a week ago and while it seems rather stable so far, the developers are all on Linux and so Windows and OS X are not as well tested and could use your help!
If you have questions you will get answers on the MegaGlest forums, or you can try to catch us on IRC (irc.freenode.net #megaglest). There's also a web chat at http://chat.megaglest.org/. I'll also get back to what you post here, of course.