Last Tuesday I went to a London Java Community talk which promised to debunk the hype around NoSQL. Whether you're already bought into a NoSQL technology, or you're just wondering what all the noise is about, it's worth an hour out of your day to see Akmal Chaudhri's comprehensive summary of the technologies out there.
Skillsmatter recorded the whole thing, as usual, so you can watch the presentation for yourself. I promise neither I nor anyone else from 10gen paid him to mention MongoDB so much.