A friend recently sent me a link to the Washington, DC DiggersList, which is kind of like a craigslist for homeowners and contractors. You can find listings for used appliances, building materials and tools, as well as listings for contractors and designers. In addition to DC, it's in about 16 other markets, and one of the goals of the founders is to keep as many building materials as possible out of landfills.
It's new, so there isn't that much up yet, but I think it could be a really helpful resource. I hope it takes off.
Update: Thanks for the shout-out, DCBlogs! (In DC Blogs Noted 5/25/10.)