Library 2.0 -- the use of collaborative online techniques as applied to libraries -- is a rather large subject, and I may well come back to it in later posts, so I'll mention a couple of ideas that hit me as useful.
But first, having read widely about Web 2.0, I'd like to make an observation about the tone of some articles about this and other new technological phenomena. Why do we (the grand "we," i.e., as a society) feel a need to diss the past? So often when some new way of doing whatever is introduced, the old way is implied to be, or even outright labeled, "bad" or "stupid." This is plain ol' not true. Blogs do not make static web pages obsolete; Twittering doesn't make blogs obsolete. Faster (the "improvement" is often speed) is not necessarily better -- sometimes it's useful, but sometimes it blurs a deeper contemplation of the subject. The move away from authoritative systems to information anarchy can give voice to neglected points of view, but it can also cause confusion. There's a place for both the old and new. Respect temporal diversity.
Tagging the catalog: If the library catalog is set up such that patrons can tag books in their own words, the books can be marked by words that are more intuitive than the standard subject terms. Furthermore, non-subject tags can be added that some patrons might find useful, such as "oversized, blue cover" or "happy ending" (yes, I realize that spoilers are controversial; managing them is a separate question). However, since this is a sort of catch-as-catch-can approach, it would also be useful to keep the "stuffy old" method of standard subject headings, either as an alternative or as core tags.
Social bookmarking: In addition to a book catalog, a library might want to keep a list of bookmarks or links, perhaps through a service like Delicious, to which patrons can add websites they want to share. To prevent abuse, the library may have to keep the option to delete sites or even to approve additions, and allow patrons to only add sites and tags but not delete or edit. Such a list would be useful in that it would contain local information and reflect the interests of the community.