You can collect the following bonuses:

Extra Life: Gives you one extra life. If you complete a level all lives collected will be available when you advance to the next level.
Double Points: You will receive twice as many points as normal after collecting this item.
Extra Time: Adds precious seconds to the remaining time.
Freeze Enemies: All enemies will become motionless for a short while after this bonus is picked up. However, touching enemies, even while they are frozen is still lethal! Amoebas will stop spreading too.