Delicious Library is definitely worth taking a look at! I've been thinking for some time (since the StoryCatcher days) of a system to let people share their books, CDs, video games, and DVDs. People's preferences and similarities in terms of media is a great social bond, and it's a shame to have that bonding element trapped on bookshelves, etc. because friends with similar interests have no way of knowing what each other has. Think of the number of book clubs in the US (thanks Oprah for mainstreaming this again) and the media sharing that happens on college campuses, etc. Sharing media is fun, cost effective, and has a definite viral revenue effect.
I could never figure out the business model for this "media library" sharing. You could do an affliate model with an Amazon or other media vendor to buy things you liked after borrowing them, but I don't think that model can support an larger, independent business. Seems like adding "community" functions to key media services is the more appropriate function, but that is something vendors will have to learn on their own.
Anyway, the Delicious Monster folks definitely have the "my library" part of this concept figured out! I'm cranking on a couple of other projects (Light a Brite, my web site, KOTR, blah, blah, blah), but I really want to play with this application!