Since I'm working on a couple of covers at the moment (and hoping to do more), I've been doing a lot of studying of great cover designs through the years. While doing my research, I spotted the Cover Browser today and thought I'd share. It's a site with a fairly good selection of great covers over the years. Scans could be slightly larger, but overall it's a great little site.
If anyone else knows of a great cover gallery site, please drop links below, I intend to build up a library of images just incase these sites go down.