Rick West let me know about a service similar to del.icio.us called Furl. He asked me what the difference was, and while I hadn’t heard of Furl before, I found a good comparison at http://blog.ultralab.net/~blogger/andy/archives/001018.html.
It looks like Furl may be for academic arenas — it allows you to save the actual contents of pages and search through them later. (Delicious only saves the URL’s.) You can also export your links in MLA format for bibliographies. Furl’s UI is full of graphics and color — perhaps the type of website most people are more familiar with.
Delicious may be more for techie arenas — it uses a minimalist interface, has an API for allowing 3rd party software to uses its services, and uses ad-hoc tags instead of predefined categories.
Having said this, when I googled for differences between Furl and Delicious, several people said they use both.