I've been using del.icio.us for quite a while now. The power of tagging immediately convinced me to make it my primary bookmark management system. The only downside to it is its interface. Being that's it's still officially in its pre-alpha stage, it's completely understandable. Fortunately, Johnvey Huang was both insightful enough to recognize an opportunity to improve on it, and intelligent enough to be able to do something about it. He calls his solution del.icio.us direc.tor. Luckily for all of us, he's also generous enough to share it with all of us.
By taking advantage the del.icio.us API, direc.tor presents users with an elegant and efficient AJAX-powered interface to your del.icio.us bookmarks. Direc.tor allows users to drill down through your bookmarks by simply clicking in tags, rather than manually crafting tag navigation URLs. It's a very impressive and intuitive piece of software.
I had some initial concerns over direc.tor's potential for efficiently managing large numbers of bookmarks, but now that my link collection numbers in the thousands, direc.tor has easily kept pace.
If you use del.icio.us, you need to try del.icio.us direc.tor.