GOG, Steam.
Used to buy a couple bundles from HumbleBundle, but I don't have and don't plan to make an account there so when it became mandatory to have one, I stopped.
I plan to add one more account - Blizzard's Battle.net - as their update to the original
StarCraft (which, among other things, made it playable offline and without running any client) rekindled my interest in the series and I don't realistically see any way the
StarCraft II trilogy will end up on GOG anytime soon. So I'll probably wait till its
"battle chest" goes on 50% sale and get it then.
As for other services... I'm too lazy to make new accounts (which means reading new EULAs, thinking of and remembering yet another username and password and trying not to mix those with the ones I already have) so no Origin ('sides, I've got
Mass Effect 1-2 and
Red Alert 3 on Steam and
Dragon Age: Origins here so all that's left there of the things which are of some interest to me is
C&C collection and it may appear on GOG eventually; even if it doesn't - oh well, no big deal) and Ubi's ULose for me.
DRM-Free is a huge plus to all the good games that I will replay time and again, especially when I'm in places with no (or mobile-only high-priced and/or traffic-limited) I-net connecton, but definitely not a requirement for games which I'm likely to lose interest in long before I might lose access to them (like
Bioshock Infinite or
Max Payne 3).