It's funny, I started to write a reply explaining my love of Delicious. But the more I articulated the reasons, the more I realized I loved what it was, and don't really use it anymore.
1. Before browsers could sync my bookmarks across multiple computers, posting them to delicious was the best way to have access to them anywhere.
2. Before instapaper let me save articles to read later, Delicious was a great way to have a tagged backlog of things to explore whenever I have freetime.
3. Before Twitter, the best way to know what your favorite developers or designers were thinking about was to follow what they were bookmarking on Delicious.
4. Before HN, looking at the usage of tags and stories per tag helped me figure out what technologies or topics were growing in popularity and find out what to read first about them.