It may be old but its fun... Monk and shaman, give monk lots of dex gear and high proc weapons and have fun! :P
Or of course like others have said, a pet class and a wanna-be healer. Maybe beastie and shaman; shaman buffs beastie's pet with dex and you go at it.
But by far the most fun combo that I've played (Friend and I) has got to be druid + necro. Necro snares, DoTs and fears, druid does same - works pretty well since druid gets some nice DoTs from different groups then necro.
Ranger + necro would also work pretty well, easier to bow kite since necro can root, fear, and snare.
Just out of curiousity, how many DoTs do shamans and necros get that stack with each other? I'd think that'd be one of the most powerful combos by far... Necro tap tanks after he DoTs, shaman heals or vice versa.