There are many Diablo I and Diablo II mods, that modify the items, skills/spells, monsters and such. Also most mods are very version specific, and most of the mainstream mods are for 1.10f (battle.net is 1.11b) but this should change soon with the release of 2 huge 1.11b Total Conversions: Shadow Empires and MetalStorm. Some of the most notable mods are Median 2008, Eastern Sun, Blackened, XyRAX, Zy-El, Back to Hellfire and Battle for Elements.