You might all know del.icio.us, the social bookmarking service. As I use this a lot and also did some research in this direction recently (see e.g. here) I wanted to try out more 🙂 Within my preparations for the Multimedia Information Systems course this semester I checked in how far LSA (latent semantic analysis) can be applied to tags and made also a small demo application. The demo fetches the RSS feed from a given user name (leave the name field blank for random names, separate multiple names by commas) and computes a co-occurrence matrix after a latent semantic analysis. Note that only the last ~30 entries are in the feed. You might then select a tag from the combo box and find the related ones.
The tool can be accessed via Java Web Start here. Drop me a line whether you like it or not.