Postal 1 has previously been available to Linux gamers after it was originally ported years ago. Ryan "Icculus" Gordon has now upgraded the Linux work on it and released it digitally to Linux users via Steam. Postal is a 3D shooter and was released originally in 1997 by Running with Sciessors.
Postal was already succeeded by Postal 2, which also has a native Linux client. The state of Postal III for Linux right now is in a mixed-up state but will likely come, eventually.
Aside from Postal 1, Dungeon Defenders was also released to Linux gamers via Steam by Gordon. He confirmed this information within a tweet. Dungeon Defenders is a tower defense / action RPG game powered by Unreal Engine 3. This title was originally released to Linux gamers as part of a Humble Indie Bundle and was the first UE3 game on Linux.