I came accross one of those top 10 video today on youtube (while I remember trying to learn more about particle filter...) and it was about the longest living creatures. I already knew lobster and tortoise were amongst the top ten, but to my great surprise the number one was a jellyfish. The video claims that they can live forever. I did find some other websites claiming that scientists agree with that, but I would like to see a proof (if there is any).

Video: http://www.youtube.com/watch?v=c1g7arWBsh0