Eschalon is a turn-based RPG (role-playing game), which tries to reproduce the feeling of classic RPG games. It's closed-source, available for Linux, Mac OS X and Windows, and it comes with a demo too. The full version is available as a download for $19.95.
The currently available version is Eschalon Book I, and you can download the demo from the official website. To start the game, uncompress the tarball and run it as ./Eschalon\ Book\ I\ Demo from a terminal or just double-click the Eschalon Book I Demo executable.
The game uses OpenGL and a native resolution of 800x600, which is also the only resolution available. It can be run in either windowed mode or fullscreen.