I've never used Steam or Desura, how is Desura? I like that they offer linux, Mac and Windows.
Desura's great. They mainly cater to the indie segment and they have quite a library of good, cheap games as well as some bigger ones from such publishers as Paradox. They do a lot of alphafunding games too - essentially you pay a very low price to get in on the alphas of games.
They have a client which you kind of need to use, although a lot of games allow you to download independent installers too. But the client is nice; it offers easy, one-click installation of games and apparently makes downloading mods just as easy, although I've never used that particular functionality. Many games also come with demos which you can also grab via the client with a click. There's also lots of community stuff all built-in too, like forums and reviews and stuff. Everything available through the client is also available on the website.
I really like Desura, and although I've never had to contact customer support I've heard they're great in that area too.
Edit: Oh, and yeah, they always support as many platforms as they can. Most of my Desura library can be played on Windows, Linux or Mac, and you only pay once.
