Something similar to steam for freeware games

Desura?

Not sure if there is any. You can try Desura which is like steam but for indie games and has some free games

I'm sure there was something like that, I gotta look.
EDIT: I was right - Tiggit
EDIT 2: oh wait

IndieCity

Maybe capsule from greenmangaming? , not sure they got there free games but maybe they got :3

uplay got some , origin

Hope it helped :)

I really don't think there is anything, best thing to do is just search for free games... I've got a list of around 80 or 100 free games somewhere on my computer that I made a while ago of things I thought looked worth playing.

steam + non-steam game overlay (you use google to get the Freeware games).
unfortunately its your only choice right now :/

Desura.

Desura.
there simply is nothing more similiar to your description as that.

jogobox or luffi maybe

There are tons of games free games you can get without a client. But I would recommend Desura, they've got quite a lot of free games on there, although beware that your not going to have Extreme fun out of Indie free to play games. Even though it's on Steam I highly recommend you trying out Team Fortress 2.

Most freeware stuff is spread over websites on the internet, so you'd have to really look. The 'Tiggit' that was linked looks pretty nice, but I'm going to assume that has nowhere near the real amount of freeware games out on the internet.

Desura