SOLDIERX.COM Nobody Can Stop Information Insemination

Free open-source software (if you want to buy you can always get the pro version)
John has been, and continues to still be, the most famous and most widely used password cracker for linux/unix systems.
Things everyone likes about it:
It's fast, it has support for cracking a lot of different but commonly used hash types, and it's able to run on just about anything.