VoltDB is an in-memory highly-scalable OLTP database that provides SQL/ACID and runs on commodity hardware in a shared-nothing cluster. It is open-source and exists in both a free/community (AGPLv3) version as well as a commercially supported version.
There is a post in the VoltDB forums discussing some of the other in-memory OLTP RDBMS solutions at http://community.voltdb.com/node/95. Feel free to review and add to that posting.
