I've posted this before, but it deserves more credit:
http://www.outpostuniverse.net/outpost2.php
This is a RTS game from 1998, but don't let the date or the simple graphics put you off. This game is extremely fun and addicting once you get past the initial complexity.
For such an old game, it still has a community, the members of which still play it online via Hamachi. It's great fun, and a really oldschool, classic RTS experience. Not to mention it's now abandonware and can be downloaded at 12mb!