del.icio.us is four today. Happy birthday del.icio.us! What can I say about it, except that I keep the sum total of my recent knowledge in there, basically anything from 2004 onwards. You get a bit of it in the sidebar on the right, but that's only stuff with the library tag, not the whole burrito.
It's a little weird that just about everything I learn and know recently can be distilled into an rss feed, but it's helped me with my memory if nothing else.