DBAN (Darik's Boot And Nuke) is designed to fully and securely wipe your disk. It's a downloadable self-contained boot disk, available for CD/DVD, floppy disk, or USB flash drive. Boot from it, and it will look for hard disks on the system, then give you the option of deleting any or all of them. (Note that for RAID arrays you may need to disassemble the array first.)
Several wipe methods are available, from a quick erase to a full three-pass wipe, including a couple that are used by the U.S. Department of Defense! The autonuke option from the first screen will automatically run through all detected disks, securely wiping them, so be careful with it.
Be warned: A full standard disk wipe, especially on an old or slow machine, can take a long time (three to four hours is quoted as standard, but it can be much more than that on older machines). I usually leave it running overnight.
DBAN is easy to use and very secure, using multiple methods (including the Gutmann method and the Mersenne twister) to ensure that data really is gone for good. It's a handy program to keep around on disk – but do make sure you label it very clearly!