Lots of good stuff to be had but here is my personal pick, it's old but for RTS style games it simply did not get any better...
Total Annihilation (Only 99p!)- http://store.steampowered.com/app/298030/?snr=1_7_7_ut2_150_7
I thoroughly recommend it to anyone who hasn't tried it before.
Great game, specially multiplayer. I got a GOG promo with all the add ons a while ago.