As it turns out, I'm a huge fan of RTS games. Although over the past 5 to 10 years I've mainly been playing FPS's such as Quake, Doom, Half-Life and Call of Duty. But there's something really rewarding about building and controlling your own army, and battling against others.
As far as I can remember, the first RTS game I bought was Age of Empires. A truly brilliant game which set the benchmark for many other games of it's type.
Next up was MechCommander. Slightly different from Age of Empires in that you didn't build any thing, you simply earned money for completing missions. But there was an art to building your mechs and choosing the correct driver which made the game enjoyable and challenging.
The last RTS I remember buying was StarCraft. Oh boy was StarCraft and truly great game by many standards and I'm looking forward to StarCraft II... when it's actually released.
I would gladly play any of the above games today. Sure their graphics might be a bit dated, but their gameplay was second to none. However, I recently came across a game called Warzone 2100, a game originally released in 1999 (and one I somehow missed) which was open sourced in 2004 and is now run by the Warzone 2100 Project. The game is similar to StarCraft in alot of respects and perhaps this is why I am finding it so enjoyable. There is a very good User Guide to help you along and if you need any help or feel like contributing to the project, there is a very active community who are willing to help.
I suggest that everyone at least tries this game out. You never know, you might enjoy it.