Candybar is probably the best one out there- I use it and I think a lot of the folks posting in the December Desktops 2010 thread do too (btw its in the Picture Gallery and is a great source of ideas- Deviantart too, for wallpapers and icons; the thread changes for every month and is pretty active).
Candybar is a paid for app but there is a trial to see how you get on with it http://www.panic.com/candybar/, there are also lots of themes about if you like that sort of thing but its not my bag- Hyperdock also might be worth a look (its actually pretty useful ) http://hyperdock.bahoom.de/