ObjectDock is probably the most widely used one. It's the one i've got on my desktop at the top of this page. It's pretty good, but make sure you've got the RAM to spare, as it almost brought my computer to a standstill, and i've had to stop using it. If you do get it, download the pro version as a torrent.
Y'z Dock is an alternative for slightly less memory use. The trade off is slightly less function and no further upgrades since the makers got sued. Google to find it