Minimum $1: Noir Syndrome, 6180, Don't Move.
More than $3: Droid Assault.
(All these are DRM-free).
NOTE: Don't Move is also linux, 6180 has a linux/mac version on the way, and and Another Perspective dev "ShaunJS" on Steam Greenlight says that about a linux port:It's definitely possible. It's on my to do list and there's no reason why I wouldn't want to do it.