Wallpapers

• Veer - your desktop will love this place and that's no wonder since Veer is a provider of visual elements and design-related products. Wallpapers are provided in several download sizes.
• Pixelgirl - home of a myriad of wallpapers that will satisfy every taste and monitor size out there.
• 555 Design - grunge style and lots of it. This guy really knows how to make your desktop dirty and his work is quite impressive.
• Sensitive Light - nature at its best, recommended for the cubicle programmers that don't see daylight very often.
• Art. Lebedev Studio - a repository of fine images. Sometimes their logo on every image bothers me but it's well worth a look.
• EndEffect - high-tech 3D future mesh-ups for the cyborg in you.
• Imagine GUI - widescreen desktops, show that shiny new PowerBook some love.
• Floral Photographs - high resolution macro flower photography.
• Mandolux- huge collections of photographs. Material suited for both larger monitors and dual monitor setups. Look for the "archive" link on the page.

Icons

• The Iconfactory - the one stop repository of the very best Mac icons I've come across. This must be your first destination.
• Brocked - Home of many icons including the "I pity the fool" Mr. T set that will make you smile every time you see it.
• Enhanced Labs - perfectly polished icons that seem to come from somewhere around 2053.
• Bombia Design - Not many icons per set, about 3, but they are closely related to a theme like Apple hardware. Very futuristic and simple, people that like a minimalist desktop now and then will enjoy them a lot.
• DV Graphics - offers a variety of icons the author developed from 1999 until today.
• LUGIC Icons - extremely stylish icons from the mind of a German designer. WOW.

Sites that offer both icons and wallpapers

• David Lanham - well known artist whose exceptional work will have great appeal to those of you that like comics.
• Xtudio Design - smooth simplistic designs, ideal for the minimalist in you.
• VanillaSoap Design - these people know their GUI skinning, recommended for the futurist types.
• InterfaceLIFT - big repository of more then 13,000 icons and 600 wallpaper. Quality stuff.
• DeviantART - they offer an extensive database of much more than just wallpapers and icons, they got it all and lots of it. Tip: use the browse options wisely to filter the search results.
• Bluburger - cartoony type stuff, bright colors.
• Xanthic Eye - futuristic, bright, good collection.

I'm sure there are more cool places to get icons and wallpapers and I'd appreciate some links in the comments as I'm eager to get my hands on more goodies :)