Not much to add here - I too field a Chaos Space Marine force, which has less options than a loyalist force, but still plays rather similar. You might want to look at the special chapters like Space Wolves and Blood Angels as well - Blood Angels are basically super aggro cc focused Marines, and Space Wolves... well, they do everything the regular Marines do, only a bit better.
Let's see, what else have we got... Orcs for starters. Very potent army in 40k, plenty of variation is possible, and there's plenty of power in the list. And they're fun too. A bit on the expensive side perhaps because most builds will have a lot of models in them.
Imperial Guard - pretty much what I said about Orcs applies as well, but the focus is on firepower while the Orcs have a close combat fetish.
'Nids are nice as well, good looking army, midfield power level. I like 'Nids.
Tau I don't like, but that's just me. Hate the models, and I never had problems steamrolling them. If one likes the aesthetic they're okay I suppose. Very shooty, absolute, and I mean ABSOLUTE crap in close combat. Possibly the greatest sissies in that field in the game. Imperial Guardsmen are practically Khorne Demons compared to Fire Warriors.
Necrons. Oh well. Not a lot of variation in the list. It looks kinda cool, but is rather weak these days.
Hope that helps.