Jesse Wolgamott presented at Lone Star Ruby Conference on four major NoSQL databases: CouchDB, MongoDB, RavenDB and SimpleDB. We've covered various comparisons before, such as one by Adrian Cockcroft. And of course there's our own article on the subject: "Is the Relational Database Doomed?" But if you'd rather kick back and watch a conference presentation, then this is for you. It has the benefit of including RavenDB in the comparison, which we haven't covered before.
The direct link for the presentation is here.