For all his heroics, Harry Potter only managed to destroy ONE of Voldemort's seven Horcruxes, and that was three years before he even knew what one was!

For those of you who aren't down with Harry Potter, here is a bit of background (SPOILER ALERT if you haven't read past the fourth book and don't want to know how the series ends) - a Horcrux is an object into which a Dark wizard has cast part of his soul. The logic behind this process is that the wizard will have achieved immortality so long as the Horcrux is never destroyed. Even if the wizard's physical body is destroyed, a part of their soul will still remain in the Horcrux, and there are magical ways to create a new body in which to house their soul fragment.

Nevertheless, this was apparently not good enough for Lord Voldemort, the most fearsome Dark wizard who ever lived. He wanted more of a guarantee on immortality, so he decided to split his soul into SEVEN different parts for SEVEN different Horcruxes! However, the major catch when creating a Horcrux is that a murder must be committed each time a wizard's soul is to be fragmented. Voldemort chose to place the parts of his soul into objects which he considered to be meaningful (and one which he didn't count on!):

1. Tom Riddle's Diary - destroyed by Harry Potter with a basilisk fang
2. Marvolo Gaunt's Ring - destroyed by Albus Dumbledore with Godric Gryffindor's sword
3. Salazar Slytherin's Locket - destroyed by Ron Weasley with Gryffindor's sword
4. Helga Hufflepuff's Cup - destroyed by Hermione Granger with a basilisk fang
5. Rowena Ravenclaw's Diadem - destroyed by Vincent Crabbe with a Fiendfyre curse
6. Harry Potter - murdered by Lord Voldemort with the Killing Curse
7. Nagini the snake - beheaded by Neville Longbottom with Gryffindor's sword

(source)