wallpapers (1024×768 minimum) is the mother load of high res flickr groups. It is chock full of images tagged with wallpaper at a minimum resolution of 1024×768. 36,669 images and counting. This will take a while to go through.

Professional Wallpapers hosts a few HD images in a number of different formats. including 2560×1600, dual monitor, HDTV and PSP/iPod formats. Nice work.

desktopography is an exibition, a showcase of nature themed desktop wallpapers created by designers worldwide. This site is awesome! great visuals and great music! Lots of great, hi def and large format wall paper and all around great design. Check them out.

Dual Screen Wallpaper is a site that features lots of cool multi monitor wallpaper. They have the following categories Nature, Abstract, Space Random and Vehicles. Most of the images are 2560×1024

mandolux This site is incredible! Beautiful multi-monitor wallpaper, in multiple formats. Most are cut into left/center/right wallpapers. There is even an rss feed for them.

IGN has a feature displaying the 50 Top Marvel Covers and a few of them are nice and large, perfect for your large screen. It sucks that they have to be in a vertical format.

InterfaceLIFT has alot of great free wideformat hi-def wallpaper. They are all sortable by highest rated, most downloaded, etc. Their images are available in sizes from 2560×1600 to psp optimized. (480×272) Awesome!

Plasma Design has several large 4:3, 16:9, dual monitor wallpaper and 30" Display wallpaper designs. They have 3 flavors Aqua, Landscapes and Abstracts. Membership (£7 fee for lifetime membership) is required to access their collection which consists of over 200 images.