http://perfectmarket.com/blog/not_only_nosql_review_solution_evaluation_guide_chart has a lucid comparison of MongoDB, Redis, TokyoCabinet and BerkeleyDB.

What's nice about the evaluation is that it mentions what use-cases which solution is likely to be a good fit.

While we're on this topic, how about a recap of Brewer's CAP Theorem (pun intended)?

And to round things up, checkout this Visual Guide to NoSQL Systems