MySQL: http://www.mysql.com/why-mysql/scaleout.html
Limitations are that it works best with read-mostly workloads. You typically have one 'master' that receives all the writes, and many 'slaves' that replicate the writes. Then you distribute the reads over all the databases.
MySQL replication is asynchronous, so you will probably have to deal with time lag problems (you write to the master, and then read from a slave before the write has been replicated).
