Some more combinations that can do wonders:
DPS
- illusionist/elementalist (tons of AoE damage)
- illusionist/ninja (leaves the barb/ninja combination far and far behind him dps wise)
- ninja/zealot (good damage because of Zealot's passive and attack speed boost)
Tank
- barb/priest (immortal, cannot die)
- barb/zealot (tanky, but also nice damage because of Zealot's passive)
- barb/elementalist (tanky and good AoE damage
- barb/warrior (tanky, but not something really special)
Other
- elementalist/shade (never run out of soul, also good for making black hole runes if you have the spellbook)
- hunter/tactician (don't know the tactician yet, but I expect that the fear and maybe cobra shot energies very well with the tactician)