First stop for your Wallpaper needs should be interfacelift.com
Why? They have a fantastic user rating and internal reviewing system which means only the best of the best user submitted content gets through to the pages you download from! Not only that, each wallpaper is downloadable in almost every conceivable mainstream resolution, so no more stretched desktops ; )
I have almost every wallpaper released by these guys stored on my computer on a rotation on my hard drive - they have a very diverse variety of mediums, and are always top notch when it comes to quality.
Here's a few I picked out to give you an idea of what they have to offer:
Really nice site guys, make sure you check it out if you're bored of your desktop!