Ballsmacker is a free virtual pool game rendered using OpenGL. Features include: Support for British pool rules, sound effects and a physics engine which scales according to rendering speed.
GTKPool is a 2D pool game using the GTK+ toolkit. It should run on Unix and Linux variants. It currently supports playing 8-ball, 9-ball and Rotation as well as a "No Rules" option for practice/just messing around. It supports many options and has configurable physics so you can do almost anything.