Achron

If it hasn't been suggested yet I would strongly recommend Achron. Available on steam or direct from the dev.

http://www.achrongame.com/site/ 

A RTS that allows you to move through time and control your units in the past/present/future. Very mindbending especially at first.

Has a great story line with a plot twist at the end.

Some levels in the campaign IMHO really aren't well balanced and you probably need to cheat to get past. 

If you want to play multiplayer I'd recommend getting it on steam, I got it straight from the dev and my understanding is that you (or one of your opponents/allies) need to do some extra work to act as the server for the game. I believe buying from steam means you don't have to worry about this.

Great sound track, poor graphics.

Admittingly metacritic gives it a 54%, not exactly a glowing review. I'd recommend you download the demo and see if its for you.

I do not regret in the least shelling out the $20 for the two copies.