If an invisible, disembodied presence entirely capable of holding(and using) a pool cue doesn't sort of freak you out, you're a much more brave soul than I am. If the fact that this disembodied presence brandishing a pool cue can actually make shots doesn't freak you out, you must in fact be a disembodied, pool-playing ghoul yourself.
Of course, I'm talking about FooBillard here, not anything spewed forth from the fiery depths of hell. You know, it's a fairly rocking game too. I suppose you have to be a pool (or snooker) fan, but if you are then there's quite a lot here to keep you busy. The A.I. (disembodied hell-soul) works very well, and even will make you think you're about to be hustled by the computer a few times. The physics, especially for an open source project, seem to be pretty damn good and the graphics are nice, shiny OpenGL. There are quite a few game modes here: Standard pool, snooker, 8-ball, 9-ball... I'm not sure if there are more. Either way, when you throw in the "two people on one computer" multiplayer, this is quite a complete package. Although the main focus for this project seems to be Linux, there are Windows and OSX ports available, plus: Even though the source may not be GPL'd, it is entirely free and open source.
Creepy soul-taking-invisibly-evil-ghoul A.I. aside, I haven't found a better game of pool (for free, no less) this side of an actual table.