MongoDB is a widely used, general-purpose database. It is open-source, written in C++, and document-oriented. MongoDB has a strong support for MapReduce and uses BSON (binary JSON) format for storage purposes.
The latest releases can be downloaded from mongodb.org.
The mailing list is very busy and almost every question on earth has been asked on there, so try searching the archives (Searching Google with
site:groups.google.com/group/mongodb-user generally works better than using the Google Groups search bar).