FEOTD (Firefox Extension of the Day): del.icio.us
If you do not know and use on-line social bookmarking site Del.icio.us, take a break now and go there. Currently owned by Yahoo, is Del.icio.us is a one of many social bookmarking tools out. Probably not the one with most features, not the one with coolest user interface - but pretty good one and certainly winner for me.
I have been struggling with the problem of how to keep my collections of URL's under control for several years. Yes - every browser has bookmarks or favorites, but if you use multiple computers daily basis (multiple means > 5), with actively accessing Web with more than single browser on each of them (usually with some subset of IE/Firefox/Mozilla/Safari/Opera), you can pretty much forget about browser's bookmarks/favorites - you will never have your bookmarks where you need them.