Cannon Brawl isn't free. Says £10.99 for me.
Also Dungeon Defenders II, Dungeon Party, Dungeonland & Dystopia are free and have achievements. Viridi has them too.
Also Star Trek Online & TDP4 Team Battle are listed on the store has having achievements. Dont know if they do or not though.
Letter Quest: Grimm's Journey is also no longer free. Louie Cooks isn't free either.
http://store.steampowered.com/app/407020/ (Star Sky - ブルームーン) isnt free either.
http://store.steampowered.com/app/403270/ (Cloudrift) also is not free.
Tom Clancy's Ghost Recon Online seems to have been removed from the store. (Couldn't find it.)
RUMP! - It's a Jump and Rump!, The Universim, Ultimate Chicken Horse, Unraveled: Tale of the Shipbreaker's Daughter, Infinium Strike, Golf Masters, Fringes of the Empire, Cityconomy, Blood Code, Angels That Kill & American Truck Simulator are not out yet and might not be free to play. (Might be worth putting games that arnt out yet with a note)
http://store.steampowered.com/app/394120/ (Atom Universe) is not out yet either so unknown if it will be free.
Altitude - Demo seems to of been removed. Cant seem to find it on the store. Empire: Total War Naval Demo also seems to have been removed. As has Football Manager 2011 Demo. HOMEFRONT Demo has also been removed.
The Stanley Parable Demo isn't unscannable. Due to a glitch with the achievement Steam will re-lock the achievement again. To actually get the achievement you need to quit the game at the right time. That's how I got it this morning.