The ads at the bottom of this site...

I love those ads. They are quite useful for sweet game deals and aren't too obtrusive.

Most of them sell games for steam (read the requirements of the game), but some games from greenmangaming require their own client, 'capsule'.
They're defenitely worth checking out.

They can offer steam codes. Greenman Gaming has its own client - Capsule. Gamersgate doesn't have a client but you download the games from them. Can't really tell you something more.

GreenManGaming has some nice deals from time to time. They often do 75% off, and then you can use a code for an extra 20% off, so yeah, check it out.

Gamersgate and GMG offer steam codes for most of the games on their store, I mostly just buy from them if they do. They will note in the discreption of the game's DRM if it require steam.

I know that GMG does have a client, gamersgate I think only offers you a download link without a client.

Green Man Gaming has capsule client for the games GamersGate I don't know but if you buy games in those sites that give you steam key you can use it on steam. I haven't bought so much on Gamers Gate but on GMG there are often good deals so it's worth using. Also if want to find deals take a look on this, there are many games site legit and good for games that you can find there, and if you are looking for games with steam key just check on the description on a game on the site you are looking, usually you'll find something like third party DRM: Steam.