Shred your hard drive
This will shred your whole hard drive - first overwriting it with random data, then with zeros. Be careful choosing the right device path! You will lose all data on the given device! The value of iterations determines the level of security. For not important data, 1 iteration is enough. For more important data 7 iterations is a good start. For the paranoid 20 iterations and destroying the inner drive platters will be a good start ;-) See the Linux manual page "man shred" for further help.
shred /dev/sdx --force --verbose --zero --iterations=1
Of course it's also possible to shred only a certain partition on your hard drive:
shred /dev/sdx0 --force --verbose --zero --iterations=1
Again, be careful when doing this!
