One that I've found works well is Iolo's DriveScrubber

http://www.iolo.com/ds/story.cfm?story=25

They offer a free trial download that you can use to make a bootable wipe floppy diskette with.
It also has DoD 5220.22 compliant wiping capabilities (will overwrite disk w/random binary 7 or greater times)

I've found it can be slow going on older drives: wipe speed depends on speed of HDD, not size of HDD

/EDIT: I forgot to mention it runs on most OSs:
Any drive, any format, any file system, any time!
DriveScrubber itself merely requires an X86 compatible processor to run (that's anything that can run a DOS operating system, from 286's on). A Windows-based bootable diskette builder (an evaluation of which is available for download) is used to easily create a floppy disk which contains its own proprietary operating system that can be run on any X86 machine by simply inserting the specially created floppy and rebooting the machine to start DriveScrubber. Once DriveScrubber is started, any floppy or hard disk can be wiped, including:

IDE, EIDE, SCSI, or floppy drives

FAT16, FAT32, NTFS, Mac, Linux, Unix, and Sun, file formats

Any operating system:

Windows 3.1, 95, 98, Me, NT, 2000, XP, etc...

Any version of DOS

Any version of Mac OS

Any version of Linux or Unix

Anything else!