Existing games on Steam with ports / expected ports to Steam Linux:
Achron - "We're very strongly considering it! However, we've had some issues getting a single binary to work on both Debian Stable and the latest build of Ubuntu at the same time." (via email)
AirBuccaneers - "For Linux version there is more work that we have not done yet."
Anomaly: Warzone Earth - "Yup, we're gonna do that, but not sure how much time we'll need for the integration." (via email)
Atom Zombie Smasher - email sent, awaiting response
Bastion - "We're looking into adding our Linux version to Steam but do not have a time estimate and cannot make any guarantees about its availability at this time." (via email to /u/elsoja)
Beep - "Yes, I do plan to release BEEP for Steam Linux. Probably sometime in the spring though as I'm currently very busy" (via email)
Binding of Isaac - "late January"
Blocks That Matter - "We are currently talking with Valve to get access to the beta Steam Client. We were busy and missed the beta launch
But I think on Monday we will have access to the client and the libraries!" (via email)
Closure - "Yeah been meaning to do that, just didn't feel like bugging the linux port guy about it while the bundle was running cause he was pretty swamped with work there."
Cortex Command - email sent, awaiting response
Defender's Quest: Valley of the Forgotten - "This is something I'm trying to get working this week."
Droid Assault - See entry for Revenge of the Titans & "Because all our titles use the same code, we only have to solve the problem once
" (courtesy of /u/iamoverrated)
Dungeon Defenders - "We're gonna talk to Valve about it so no guarantee/details/etc about it happening or not, right now."
Fieldrunners - email sent, awaiting response
Gish - email sent, awaiting response
Hacker Evolution: Duality - "Yes, we are working to deploy Duality for Linux STEAM
" (via email - /u/zainin), "Also, making exlusive version for big screen mode" (via email)...now in Store
Hacker Evolution: Untold - "We are working on it right now. It will be out as soon as possible and not later than the end of this month." (via email)
Jamestown - email sent, awaiting response
Legend of Grimrock - email sent, awaiting response
Little Inferno - "We will be providing Little Inferno to both Linux and Mac users, but no ETA announced yet" (via email)
Lone Survivor - email sent, awaiting response
Lugaru HD - email sent, awaiting response
Lume - "... as we are currently in process of porting Lume to the new Linux shop window on Steam. It should be a matter of weeks until it's up there..." (via email)
McPixel - email sent, awaiting response
Nuclear Dawn - "After this we will move onto 7.x which will bring Linux client support to Nuclear Dawn." (info from /u/iamoverrated)
Offspring Fling - "I would say its about 50/50 right now...I need to change how the final application is built. I basically need to wrap Offspring Fling with software to make it into a Linux executable, but currently I'm having some problems getting it to work for unknown reasons... until I can work out these bugs, I can't get the Linux version onto Steam." (via email)
Revenge of the Titans - "Yes, when we've got a bit of time"
Shadowgrounds - " We will probably bring the other games to Steam Linux at some point, at least Trine... but I can't promise it just yet so no ETA, there's some other issues involved (we didn't port them ourselves and they have a bit too many technical issues compared to Trine 2). But it will most likely happen at some point.", "the SG games are a bit 50/50, we sell them elsewhere but they bring a lot of support issues in their current state so we're hesitant to broaden the audience too much if we can't get the issues fixed... But we'll see how that goes." (via email)
Shadowgrounds: Survivor - See Shadowgrounds entry
Shank 2 - "I do believe we plan to make the Linux version of Shank 2 more available, however I don't have an ETA at this time. I will check that out again today and see if I can push things along." & "Btw, it looks like it could be coming at any time, I just don't know when that is. Hopefully sooner than later. "(via email)
Shatter - "Eventually - our guys are pretty tied up just at the moment, but that's the plan."
The Cave - "Plus... Mac and Linux on Steam Play!!" (picked up by /u/kamjau)
Titan Attacks - See entry for Revenge of the Titans & "Because all our titles use the same code, we only have to solve the problem once
" (courtesy of /u/iamoverrated)
Torchlight - email sent, awaiting response
Toki Tori - "About Toki Tori 1 Linux, yes we are definitely going to bring that towards Steam Linux, but we are still not sure when" (via email)
Trine - See Shadowgrounds entry & "I think Trine is a given, just needs some time (i.e. spring)" (via email)
Vessel - "We will! Gonna fix some multithreading issues first so its fast though." (via email)
Waking Mars - "I think we're clear to submit the Linux version but our Linux programmer has been busy with other stuff. I hope we get it into the store for next week!" (Posted Dec 14th!)
X3: Reunion - Tweet tweeted awaiting response about plans for their re-port going to Steam
Zen Bound 2 - email sent, awaiting response
...
Greenlit Games - these are games that have been given the go ahead by the community and I assume a Steam Linux version (as greenlight is a Steam thing):
Asylum - Topic restating Linux support (note: release date "Halloween 2013")
Eador. Masters of the Broken World - Announcement on Linux and Mac Support (note: release date "Q1 2013")
Euro Truck Simulator - “Finally today we see the light at the end of the tunnel - Euro Truck Simulator 2 Linux port is almost finished. Very soon we should be able to start beta testing it.”
Folk Tale - Article on the game, including their support for Linux (Demo and beta expected "Q1 2013")
Gear Up - "To be released for Windows, Mac and Linux" (Release date "Spring 2013")
Interstellar Marines - Steam Linux support confirmed on their greenlight and kickstarter pages (note: release date "Late 2013)
Kentucky Route Zero - Linux version "shortly following" Windows and Mac version (note: have released first 'act' very recently)
Kinetic Void - "We will support Linux, and we can build out it, however we have not done any testing yet. If everything works fine as is, we will provide it along with the other releases otherwise we will need to do some fixes and this would delay the linux version. Linux support for Unity is still very new." (note: scheduled for release in "Q1 or Q2 2013")
Leisure Suit Larry in the Land of the Lounge Lizards: Reloaded - Announcement about full support for Linux (note: scheduled for release " Late 1st Quarter 2013")
No Time To Explain - email sent, awaiting response
The Stanley Parable - "I'm going to try to get a Linux version to be released with the Windows and Mac versions. If Valve don't port the Portal 2 branch of Source to Linux in a timely manner, I will do so myself." (note: release date "TBA 2013")
War For The Overworld - Initial announcement of Mac & Linux support though linux support is stated on their Greenlight and Kickstarter pages (note: release " August 30th, 2013")
....
Note regarding Games awaiting Greenlight approval
Also see Steam Greenlight - last time I checked there were 200+ games that were hoping to get Greenlit that stated that they'd be supporting Linux. There's ones that have clearly done little development and others that just look awful, but lots of good stuff in there too (e.g. Receiver, Fotonica, Legend of Aethereus, Monster Minis Extreme Off-Road etc etc). Obviously, you can support those games that seem good by voting them up on greenlight (note: it is possible to filter for just games with linux versions stated)
...
Existing or greenlit games where the news ain't so good - Super Meat Boy, Incredipede, Spirits, Wizorb ("we have no plan to release Wizorb on Steam" via email), Amanita-design games (Machinarium, Samorost, Botanicula) - "well, we do think about it but right now we are too busy. I am sorry." (via email), Avadon: The Black Fortress ("It might happen someday, but it would require a bit of work on our end and we-'re swamped with Avadon 2", via email)
...
If anyone has any more information about existing Steam games that are awaiting Steam Linux versions, then please do say and I'll add.
Edit (15/01/13) - added Beep, amended Shank 2, credited Bastion properly, made distinction between awaiting greenlight approval and greenlit more clear, added new Greenlit games (LSL, Eador, Asylum, WFTO), added The Cave, Removed Postal 2 (it's purchasable for linux on Steam now!), added Titan Attacks / Droid Assault & Nuclear Dawn.
Edit (16/01/13) - put amanita-design games in not-so-good category, added Interstellar Marines, added entries for avadon and achron.