Power Bombs grant you a devastating Morph Ball weapon, lay one of these babies and any enemies on-screen are history. You can also use Power Bombs to break through Power Bomb blocks. There's only 9 Power Bomb Tanks to be found, giving you 18 bombs in Easy and Normal but only 9 on Hard; use them wisely.
Spoilers abound below as you can't find any Power Bombs until after defeating Mother Brain. This also means that you'll be able to find all 9 of them straight away without requiring any extra items.
Samus' upgrades | Missile locations | Super Missile locations
Power Bomb locations | Energy Tank locations