I wasn't bothered with downloading another online game, back last year when my connection sucked. Now mine's awesome, but I have a shameful 40 gb bandwidth cap, so I only play one online game.
Combat Arms is one of the best free MMOFPS games out there, though it's riddled with hackers, which is its main drawback.
True MMORTS games are quite hard to come by. I suggest going with RTS titles with strong multiplayer components. The ones Krizaz mentioned are a few examples.. Then there are those browser based games, which are mostly in text and graphics, though I don't think you'd like them.
I recommend Age Of Empires ... Evony is crap. Any games like Age Of Empires are usually good . Rise Of Nations is a good game too. You evolve through time eventually getting to the Military Age.
Another favorite of mine is Command & Conquer. Though there are a sure lot of those games.
I've never played AoM. I'll sure do that soon, since I'll have to pick up a copy. MacSoft is the game's publisher for Mac, as Microsoft Game Studios published the game, made by Ensemble Studios.
You might be able to find some good custom content url=http://aom.heavengames.com/]here[/url]. Though I've only played AoE II, so I have only downloaded scenarios for that game, here.
