Firefox only (Windows/Mac/Linux): Firefox extension Bookmark Previews creates thumbnail previews for all of your Firefox bookmarks. Once installed, you can browse your bookmarks in a straight thumbnail view or in an iTunes Cover Flow-esque album view. Apart from the impressive eye candy, Bookmark Previews could be an especially handy tool for organizing and cleaning out your bookmarks, since rather than inspecting each link to determine what it’s pointing to, you can just check the thumbnail to determine whether or not you need it. Bookmark Previews creates previews each time you visit a bookmark, but since your previews will be empty when you first install it, it can also create previews for all of your bookmarks automatically in the background. Bookmark Previews is free, works wherever Firefox does.