Released in 1999, brought to open source by petition and still being improved upon, Warzone 2100 is still a strong RTS! It is a worthwhile mention to any RTS fan and requires no investment to try.

Originally Posted by wikipedia

Warzone 2100 is a real-time strategy and real-time tactics hybrid computer game, developed by Pumpkin Studios and published by Eidos Interactive. Although comparable to Earth 2150 in many significant respects, it does contain certain unique aspects, that clearly differentiate the two, including various radar technologies, a greater focus on artillery and counter-battery technology, more frequent in-game cinematic updates as gameplay progresses, and customizable vehicle designs. It was released in 1999 for both PC and PlayStation. On December 6, 2004 the source code and most of its data was released under the GNU General Public License, thereby making it a free game. On June 10, 2008 the license of the game was clarified, loosened and distribution of movies and soundtrack was permitted.

Source: http://en.wikipedia.org/wiki/Warzone_2100

7 great reasons to try it!

1. First and foremost, there are no preset units. You research the parts and you decide how your units are assembled. This is typically done by choosing a type of tread, wheels, hover platform, wings, body, and turrets. You can even can name your units and save the set for another game.
2. Massive research tree! So much so that you wouldn't possibly research everything in the course of a single game.
3. While not themed with races, the game allows you to develop your adhere to your own type of RTS play style. The parts in the game can lead to tanks, flying units, cyborg infantry and an array of fortifications.
4. Full 3d view of the battlefield: panning, zooming, rotating.
5. While the graphics are not cutting edge, recent developments have cleaned them up. In fact at 1920 x 1200 the units have clean lines, shadows and perfect framerates.
6. Allows up to 8 players for multiplayer and contains a single player campaign.
7. It is absolutely free, open source, and works with plugins.

Tech Tree: Clicky

WZ2100 Resurrection Website: Clicky
Download page: Clicky

Grab the Latest stable release / 2.0.10 / 2007-12-28

Any questions please post here and I will be happy to help you.
Thanks to jsnipy from Trackr for parts of his original post.