There's been a lot of comparison of SQL and NoSQL lately, but what is to be done if you're already sold on NoSQL and just don't know which database is for you, or your project in particular? To meet that need, Kristof Kovacs has assembled a rather thorough comparison of NoSQL databases. In fact, he covers the following:
For each database, he provides an overview - the language it's written in, the big selling point, the license, the protocol - as well as bullet points on the various features provided. Also, he summarizes the best reason to use each, as well as an example of an ideal project.
There's a lot of great information here. The only potential drawback/advantage is that it's not dated. Instead, it's regularly updated, though only parts at a time. In other words, features for some databases may be out of date, while others are up to date - you can tell by the version number he provides.