Let's compile one here.
Everyone post the ones they know about! (I am posting without double checking. If I made a mistake please point it out.)
Here are a few:
Fire + Water = Steam.
Fire + Poison = Explosion.
Fire + Oil = Bigger explosion.
Air + Water = Electrified water.
Air + Blood = Electrified blood.
Air + Steam = Thunder cloud.
(The above three conduct electricity through each other.)
Might post more after some experimentation.
