Singleplayer: Scientist Slaughterhouse. Extra firing modes, even a 4th one on the crossbow. SMG can do stuff like fire napalm nades, plasma nades (no... not halo ones). Rocket can do stuff like home by itself and change warheads. The SWORD (^.^) can cut sci's in half every now and then. The shottie has explosive rounds and stuff... grenades are able to do about... like 6 modes total, crossbow can make a net of lasers that hurt enemies, or shoot explosive darts, there's a laser pistol that can send enemies flying, drilling into the ground and dieing, or floating up and gibbing, and even more stuff. AKA: Best weapons mod evarz along with a singleplayer campaign full of funny traps to kill with.
http://www.planethalflife.com/ssh/Multiplayer: Didn't get to play Sven Coop really so I'd say DoD or CS.