A popular in-memory database system is McObject's eXtremeDB, see
http://www.mcobject.com/extremedbfamily.shtml
There is also a good white paper, offered for download from the home page of McObject's Web site, called, "In-Memory Database Systems: Myths and Facts". It clarifies some questions that arise when discussing the technology, such as, "how is an IMDS different from a 'traditional' database running on a RAM disk?"