Groupees unlocked the extra game: Kaptain Brawe: A Brawe New World link.
That's a really kewl adventure!

The other games:
Zero Gear, Revelations 2012, Guns of Icarus, Metal Drift, Laxius Force

Link: Bemine 4

It's still waiting for the mystery bonus game, but for 1$ you already have a bunch of good games!

Edit: Kaptain Brawe is for every 1$ dollar purchase (goal reached).

Edit2: They changed the 3th achievement into Battlemass PC + MAC (Desura Key + DRM-Free). Apparently Brawe is going to do a sales, so they asked to withdraw the bonus...