We dont know if there'll be a Valve game released next week.. I want L4D/L4D2 more than anything else though!
But games by other publishers I expect to see soon - Penumbra trilogy: it was briefly available to partial download on the Mac, then vanished completely from the library, much like CS:S and TF2 have been missing while in their betas. Penumbra already has Mac versions so I don't see why it would really need much beta time.
At some point we'll be seeing Plants vs Zombies (probably coinciding with L4D if Valve/popcap can time some sort of zombie day together)
I'm thinking the next release will probably be CS:S, with L4D / L4D2 coming soon afterward.
What I really want now though, besides L4D2, is Audiosurf, Fallout 3, Unreal 3, GTA series, and Mirror's Edge. Any one of those would make me hugely happy.