Someone had suggested that we needed a big-huge thread for wallpaper (that could possibly be stickied), and I am OMG-bored at work today, so I figured I'd give it a shot.
Many thanks to spotdog14 for starting the widescreen wallpaper thread from which I am going to rip a bunch of links.
Post any cool sites you find and I'll add them to the list as I'm able. Make sure to take your popup blocker with you, as some of these sites are....questionable. More details on each site in V2.0
Google
Google will allow you to search for any size wallpaper. Simply enter the dimensions(1920x1200 for example) into the search box and you can find whatever you need. I haven't found it to be 100% effective, but it works pretty well.
http://images.google.com/
Standard(4:3 & 5:4)
http://browse.deviantart.com/wallpaper/
http://www.mandolux.com/
http://wallpapers.jurko.net/
http://interfacelift.com/wallpaper/index.php?w=0&h=0&sort=date
http://www.plasmadesign.co.uk/43selection.htm
http://www.digitalblasphemy.com/
http://www.skins.be/index.php
http://www.bfarber.com/
http://www.carwalls.com/
http://exoteric.roach.org/bg/index.html
http://www.seriouswheels.com/
http://hubblesite.org/gallery/wallpaper/
http://www.coolwallpaper.com/
http://www.wallpapervault.com/
http://www.animewallpapers.com/
http://www.galttech.com/wallpaper.shtml
http://www.yourwallpaper.com/
http://www.caedes.net/
http://www.cognitivedistortion.com/
http://www.desktopia.com/
http://www.visualparadox.com/
http://www.backgroundsarchive.com/
http://www.best-wallpapers.com/
http://www.digitaldesktopwallpaper.com/
http://www.desktoppictures.com/
http://www.wallpaper.net.au/
http://www.themexp.org/listings.php?type=wp&view=date
http://www.themeworld.com/walls/index.shtml
http://wp.madcowdisease.org/gp
http://www.vladstudio.com/home/
http://www.themesunlimited.com/catFrame.asp?MID=9
http://www.pc-wallpapers.co.uk/
http://www.desktopwallpapers.net/
http://www.veer.com/ideas/wallpaper/
http://www.1000pictures.com/
http://www.basehead.org/
http://www.joeprice.co.uk/wallpaperjoe/php/index.php
http://www.freedesktopwallpapers.net/
http://www.wincustomize.com/Skins.aspx?LibID=8 - Great for every aspect of GUI customization
http://www.customize.org/list/wallpaper - Great for every aspect of GUI customization
http://www.skins.be/index.php
http://www.dieselstation.com/
http://3dcrew.com/ (new)
http://www.macdesktops.com/ (new)
Widescreen(16:9 & 16:10)
http://browse.deviantart.com/wallpaper/widescreen/
http://www.mandolux.com/
http://wallpapers.jurko.net/widescreen
http://interfacelift.com/wallpaper/index.php?w=0&h=0&sort=date
http://images.google.com/images?q=1680+X+1050&hl=en&btnG=Search+Images
http://www.plasmadesign.co.uk/
http://www.digitalblasphemy.com/
http://www.skins.be/index.php
http://www.bfarber.com/
http://www.carwalls.com/
http://exoteric.roach.org/bg/index.html
http://www.seriouswheels.com/
http://hea-www.harvard.edu/~fine/Tech/desktop-images.html
http://wp.madcowdisease.org/gp (only a few here)
http://www.vladstudio.com/home/
http://www.veer.com/ideas/wallpaper/
http://www.skins.be/index.php
http://www.macdesktops.com/ (new)
Multi-Monitor Resolutions
http://www.csbmb.princeton.edu/~smcclure/dualmonitor.html
http://www.mandolux.com/
http://exoteric.roach.org/bg/index.html
http://www.vladstudio.com/duals/
http://www.plasmadesign.co.uk/dualmonitorselection.htm
http://www.ihateaol.co.uk/misc/dual_monitor_wallpapers/
http://www.dualscreenwallpaper.com/
http://browse.deviantart.com/wallpaper/multidisplay/?view=1&order=5&limit=24
Many thanks to spotdog14 for starting the widescreen wallpaper thread from which I am going to rip a bunch of links.
Post any cool sites you find and I'll add them to the list as I'm able. Make sure to take your popup blocker with you, as some of these sites are....questionable. More details on each site in V2.0
Google will allow you to search for any size wallpaper. Simply enter the dimensions(1920x1200 for example) into the search box and you can find whatever you need. I haven't found it to be 100% effective, but it works pretty well.
http://images.google.com/
Standard(4:3 & 5:4)
http://browse.deviantart.com/wallpaper/
http://www.mandolux.com/
http://wallpapers.jurko.net/
http://interfacelift.com/wallpaper/index.php?w=0&h=0&sort=date
http://www.plasmadesign.co.uk/43selection.htm
http://www.digitalblasphemy.com/
http://www.skins.be/index.php
http://www.bfarber.com/
http://www.carwalls.com/
http://exoteric.roach.org/bg/index.html
http://www.seriouswheels.com/
http://hubblesite.org/gallery/wallpaper/
http://www.coolwallpaper.com/
http://www.wallpapervault.com/
http://www.animewallpapers.com/
http://www.galttech.com/wallpaper.shtml
http://www.yourwallpaper.com/
http://www.caedes.net/
http://www.cognitivedistortion.com/
http://www.desktopia.com/
http://www.visualparadox.com/
http://www.backgroundsarchive.com/
http://www.best-wallpapers.com/
http://www.digitaldesktopwallpaper.com/
http://www.desktoppictures.com/
http://www.wallpaper.net.au/
http://www.themexp.org/listings.php?type=wp&view=date
http://www.themeworld.com/walls/index.shtml
http://wp.madcowdisease.org/gp
http://www.vladstudio.com/home/
http://www.themesunlimited.com/catFrame.asp?MID=9
http://www.pc-wallpapers.co.uk/
http://www.desktopwallpapers.net/
http://www.veer.com/ideas/wallpaper/
http://www.1000pictures.com/
http://www.basehead.org/
http://www.joeprice.co.uk/wallpaperjoe/php/index.php
http://www.freedesktopwallpapers.net/
http://www.wincustomize.com/Skins.aspx?LibID=8 - Great for every aspect of GUI customization
http://www.customize.org/list/wallpaper - Great for every aspect of GUI customization
http://www.skins.be/index.php
http://www.dieselstation.com/
http://3dcrew.com/ (new)
http://www.macdesktops.com/ (new)
Widescreen(16:9 & 16:10)
http://browse.deviantart.com/wallpaper/widescreen/
http://www.mandolux.com/
http://wallpapers.jurko.net/widescreen
http://interfacelift.com/wallpaper/index.php?w=0&h=0&sort=date
http://images.google.com/images?q=1680+X+1050&hl=en&btnG=Search+Images
http://www.plasmadesign.co.uk/
http://www.digitalblasphemy.com/
http://www.skins.be/index.php
http://www.bfarber.com/
http://www.carwalls.com/
http://exoteric.roach.org/bg/index.html
http://www.seriouswheels.com/
http://hea-www.harvard.edu/~fine/Tech/desktop-images.html
http://wp.madcowdisease.org/gp (only a few here)
http://www.vladstudio.com/home/
http://www.veer.com/ideas/wallpaper/
http://www.skins.be/index.php
http://www.macdesktops.com/ (new)
Multi-Monitor Resolutions
http://www.csbmb.princeton.edu/~smcclure/dualmonitor.html
http://www.mandolux.com/
http://exoteric.roach.org/bg/index.html
http://www.vladstudio.com/duals/
http://www.plasmadesign.co.uk/dualmonitorselection.htm
http://www.ihateaol.co.uk/misc/dual_monitor_wallpapers/
http://www.dualscreenwallpaper.com/
http://browse.deviantart.com/wallpaper/multidisplay/?view=1&order=5&limit=24