del.icio.us is a social bookmarking service. Essentially, instead of keeping all your bookmarks/favorites in your browser, you use a bookmarklet to quickly post them onto your public delicious bookmark site. Others can see them if you wish, and you can access them anywhere, and even post the rss feed to your website.
I'd put off playing with this for quite some time. But it seems that, like everyone else who has tried it, I'm hooked. del.icio.us forever!