Hot flash: frostbite/icestorm(or weapon with freeze proc) + firestorm. This is the only self combo of a mage.
Together with other classes there is also nature strike: Archer roots the enemy + lightning.
And Terror: Nightmare + hellscream (by bear)
Mages are also involved in all of the three mega combos, but these usually occur at random and are not really coordinated.
Thats all i know off the top of my head, I'm not that familiar with mages. :P
There needs to be a little gap between both skills for the combo to work (about 1/2 - 1 sec)

Hope this helps