Drawar

Have a browse matey:

http://del.icio.us/nilsgeylen/wallpapers

Basically any place that has visually stunning desktop backgrounds. Seriously, they don't have to be Mac specific. Hit up Google and you should find some good stuff.

I've found heaps of great desktop images here: http://interfacelift.com/wallpaper/

Veer (http://www.veer.com/) has some great ones as well.

If you're into anime then AnimePaper is where to go. Even if you don't care for anime, some of them are really visually stunning.

VladStudio is where I get most of mine.

I tried posting this earlier but it seems to have been lost. I have too many browsers and tabs open at work...

Just a response to Scrivs: I personally like Mac-ish wallpapers for my Mac. Not sure how to define that, but some just won't do.

Weird?

a giant stylized windows logo, for instance.

I do know what you mean, though. There is just a certain aesthetic that can work in windows, but simply can't on a mac os. I'd say that applies back as far as system 7.

InterfaceLift has been the best resource that I've found for backgrounds.

I love the wallpapers over at Veer.