Block Breakers are machines in RedPower which, when pulsed with a redstone signal, will break the block in front of them, even other machines from RedPower2. Put Pneumatic Tubes or a chest behind it to take the item out.
This machine can be very useful for many purposes, such as cobblestone generators, or collecting obsidian as it breaks any block instantly, including obsidian and end portal block. Crafting the Block Breaker gives you the "OM NOM NOM!" achievement.