Linear: which is fun to play with if you are using lasers i would suggest this , if using rapid fire play with the emissions of this one.
Parabolic: Basically used for arches like mortar's , arrows , catapults, and cannons.
Spiral: Kinda pointless really , but you can make it go really fast and make a sonic cannon out of it.
Don't forget about the SPLASH: This is very fun to play with considering that you can mess with the emissions and which direction the splash happens , i make a wave of energy with this effect very cool.
Thats all i got