If you're into quick tactical RTS:
CoH ? (most like it, even people that usually hate RTS )
Big scale strategic RTS:
Supreme commander:FA ( some love it, some hate it, I'm hooked personally, it needs a lot of ram, cpu power, and gpu power though, it pushes every system to it's limits...) .
Or get Civ4 + Civ4 BTS exp. pack if you think turn based might be something for you, again you migth hate it, but if you like it, you'll love it and it's more addicting than drugs.
Try the LFS demo perhaps if you like Sim racing, then you can buy the S2 if you like it.