I have a real tough time finding quality backgrounds for my desktop. I am still in the hunt for that ever elusive site that is just loaded with tons of designer backgrounds.
Mandolux is one of my favorite sites for high-def desktop backgrounds. I'm using this Buddha one on my myspace page. This photographer has an amazing collection of photos that are free to download. With a variety of resolutions and even multiple screen layouts, this site is amazing!
I love Veer's collection of desktops as well. Whitty, whimsical, modern design. This site is graphic design based so there's lots of goodies. There's tons of art you can browse as well. Good stuff. You have to register to view and download their backgrounds but it's free and well worth it.
deviantART and Interfacelift are hit or miss. There are so many artists and endless wallpapers to sift through, it can get very daunting. I don't know if I've found one yet but I feel like one day I will hit gold and find that one that I just love.
I found some really good ones that are great for the holidays. These wallpapers from tsk tsk's blog are great. I'm using the wreath one. They're not kitchy or cheesy but have enough of a christmas reference to brighten up your desktop for the holidays.