There are actually two games called Slime Volleyball: The original mentioned in dave2's writeup, which is primitive and has very dodgy physics, and Quin Pendragon's completely rewritten, 2-player version of it, which is better known and has become something of a classic since its release in 1999.
Many Slime Volleyball mods exist, largely due to the fact that the game's Java source code was made available on Quin Pendragon's web page in 2001. Apart from proper mods of Slime Volleyball, there are also games such as SoccerSlime (by Quin himself), Slime Dunk Basketball, and others, which are about as close to their respective sports as Slime Volleyball is to volleyball.
Recently, Slime Volleyball has been independently remade in 3D twice: Once in Java by Adam Shaw, and once for Windows by me. First, let me say Adam's game is very cool. It's got very lively gameplay, although it departs somewhat from the regular slime experience, requiring movement in all three dimensions. My version, in contrast, is essentially the same 2D game remade as a conventional Windows game with 3D visuals, sound, and more control options (in future versions). Also, it's now free software released under the GPL.
http://fractal.leet.net.au/ - Quin Pendragon's site, with links to several Slime Volleyball games.
http://www.ultimateslimegames.tk/ - a comprehensive slime games site, and the only one which has linked to my site thus far.
http://motiz88.googlepages.com/slime3d.html - My game, Slime Volleyball 3D.
http://www.3dslime.com/ - My excellent Java-based competitor :)