Just reading and thinking:
How could we make better use of the integration of folksonomiesM and user-based vocabularies?
Why can't our catalog let users find items of interest and then store them for later retrieval using their own tags. Take a look at this Flickr page for architecture. Notice the "related" and "see also" links? The same thing happens on the del.icio.us page for architecture. Imagine the display of this type of folksonomy integrated into a library's catalog, so that users could find titles and subjects for "architecture," but they could also browse by tags (such as "buildings" or "urban"), which they could then bookmark themselves and specify as "public" or "private" (like Furl's "private archive" feature). Aggregate the public tags and let users access their private ones.
What if records retrieved from structured OPAC search results displayed those types of user-based tags alongside the MARC data? (It would be the best of both worlds but the ILS vendors will probably complain.) Then, if a user is really interested in a particular topic, he could subscribe to the feed for the standard subject search, the aggregated user-generated public tag, or a combination of both.
Another idea: add visual "what's popular" and "what's recommended" pages like these to our catalogs. We could also push this via RSS.
Now take this a step further and apply it to reader's advisory. It's pretty obvious that users like sharing their own thoughts and information. Why not take advantage of that? Imagine the read-alikes and recommendations users would build using their own folksonomies! Let them tag cozy mysteries and robot sci fi and financial nonfiction titles how they want to find them. And if you really want to get social about the whole thing, how about building out those tags cross-catalog? What would WorldCat look like if tagged by users, especially now that OCLC has opened it up?