del.icio.us seems to be enabling a spate of innovative experiments. extisp.icio.us generates a map of a user's "tagspace". Buzz Andersen cooked up a Mac OS X desktop client for your social bookmarks. Delicious Mind is Ian Oeschger's hack to take your del.icio.us posts and, through a Java applet, turn them into a mind map. Not to mention a number of bookmarklets that provide more information before posting.
del.icio.us's REST API lends itself quite nicely to these types of explorations. The only major issue that I can see is that del.icio.us is pretty HTTP caching unfriendly. Cookies are always set and none of the useful HTTP headers are set. This makes it a tad difficult to write a truly friendly client and for now people are trying to be nice.
I wonder if the other social bookmarking services are seeing any of this type of hackery? Maybe I should just put this blog full time on the social bookmark beat.