I was sent in Gearend [Official Site] by a reader, as it not only looks like a pretty good 2D exploration game, but it's also developed on Linux.
The developer is accepting beta applications, so fill this form if interested.
About the game
Gearend is a small 2D non-linear exploration game in production. You control a small robot named Epsilon-98, or "Eps", as he explores an abandoned robotics research facility. Having been awoken by unknown forces, he sets out to find out who has awakened him, and determine just what exactly has happened to the facility and its inhabitants. Find and equip arm, leg, and core parts to help you fight, move, and navigate as you explore the sprawling remains of an advanced robotics research facility.
It's made by a single developer.
The game has been greenlit, so it's coming to Steam.
Thanks for letting me know, aFoxNamedMorris!