I think Redis is one of the easier NoSQL databases to pickup, because it's easy to install and is mainly just a key-value database with some bonus features. Your entire DB is stored in memory and persistence is done by asynchronous writes to disk, so it is possible that you could lose data if something were to reboot for instance, but it also means that it's really fast.

Simple twitter clone with Redis: http://code.google.com/p/redis/wiki/TwitterAlikeExample