I've always been asked at work at my current job and my last job where I find good wallpapers. One has to be careful these days, most free wallpaper sites are run with malicious and intrusive ads, I shall call them ad ware. I have a couple sites that have ads, everyone needs ads to host this massive amount of bandwidth, but they are both non intrusive sites.
The first one is one that I found on Digg a couple years ago, and I've always stuck with it. Interfacelift has always drawn my attention mostly because it's not full of wacky photoshopped super geeky and cartoony wallpapers. It's mostly just clean shots of nature and urban life. My favorite part about interfacelift is the variety of sizes for use, they even have HD which looks really good on any LCD monitor. They also have icons and themes for those of us who are tired of the default graphic user interface in Windows and OSX.
The other one is new to me, it works almost like a social network, where the other one just works for me as an RSS feed. SocWal also has the many different types of wallpapers, from standard, to widescreen, to dual monitors, but it has kind of a thumbs up or down system. I have found breathtaking wallpapers on this site and I tend to go here first now because it's updated so much more rapidly than interfacelift.
Enjoy them both, feel free to show me the wallpapers and wallpaper sites you're using too.