As reference of cassandra, mongoDB is a kind of No-SQL database based on JSON documentation storage.mongoDB Main Release is based on C++ implementation, feature lists:
1, Document-oriented storage » JSON-style with dynaimic schemas
Schema Design run on top of BSON format.
2, Full Index Support » Indexes enhance query performance, often dramatically.
3, Replication & High Availability » Master-Slave Replication & Replica Sets
4, Auto-Sharding » scales horizontally via an auto-sharding architecture.
5, Querying » fast query based on Query Expression Objects, and multi-external Query language interface support – Java, Perl, PHP, Python, Ruby, C# etc
6, Fast In-Place Updates » MongoDB supports atomic, in-place updates as well as more traditional updates for replacing an entire document.
7, Map/Reduce » batch processing of data and aggregation operations.
8. GridFS » specification for storing large files in MongoDB.

As a very simple start, I downloaded via Git source control of mongoDB into my virtual ubuntu 10.10 system [/home/app/08_mongoDB]. Using 1.8.0 stable version.
1, the building reference could be found as following link : Buiding for Linux
2, Before you try to start mongoDB, please check with Quickstart with Unix.
A little bit difference with traditional Database installation, the major db file location should be manually created as linux user access right.

In principle, as source code is avaiable, /home/app/08_mongoDB/mongo/version_20110320.txt could be considered as the version checking-up documentation and good example of distributed database implementation.

Take care! I preferred to cassandra as major open source example. Regarding mongoDB, it will be a good references when the topic – How multi-language will be supported.