There's a lot of ways to do it, but they all do the same thing.
In bash:
$~ shred -zn10 /dev/sda
That'll securely erase everything on block device /dev/sda--give it a while to run as it's writing random numbers across the entire drive and then finishing by writing nothing but 0s on it. This makes all data on the device non-recoverable.
You need to overwrite the data anywhere from 4 - 15 times before it's clean and nothing can be recovered from it.
That's essentially all dban/wipe is doing. If you want to get even more primitive, then you can use dd (garunteed to be on all *nix systems)
$~ dd if=/dev/random of=/dev/sda && dd if=/dev/zero of=/dev/sda
That's the same as doing one pass, but if shred is there (and it usually is) then it'll do all 10 passes for you. I guess you could just throw that dd command in a simple loop:
$~ for i in `seq 10` ; do dd if=/dev/random of=/dev/sda && dd if=/dev/zero of=/dev/sda ; done
Dban or wipe will do all this for you, but you can do it yourself.
(Note, don't do it on the currently-running OS drive, because it'll eventually erase glibc.so being used to do the overwrite. If you want to do it on multiple drives, just plug them all into the same computer, and run shred on all of them from a live-cd of your chosing)