Sega (I know it's a publisher) has a few of there titles either all ready ported to Linux or is going to port to SteamOS (Total War: Rome III)
The Metro series is being ported - which while 4A Studios isn't a huge name, their publisher (Deep Silver) definitely has some AAA titles under belt (Saints Row to name the top one).
Of course Valve has ported some of their titles, and more importantly the Source Engine. I'm sure their full catalogue will be ready to go soon. Don't underestimate that - Counter Strike is still one of the most played online games going, and so it DotA now.
One last one I'll include is Brutal Legend - yes it's old. Yes it's wasn't wildly popular - but it's another engine successfully ported.
Yes - I know this is a small sample. And like I said in an earlier post - I wonder about critical mass. If anyone can pull it off though, it is Steam. I remember Loki Games trying to achieve critical mass - they ported games like Myth and Railroad Tycoon and Heretic. I still have about 10 of their ports. Unfortunately - they didn't get critical mass - and most importantly they didn't get first day titles to release. The back library will be important for SteamOS/Machines - but even more important will be the ability to get AAA titles released on par with the big 3 (Sony, MS, Nintendo).