Gem Stone is an Object Oriented Database that completely owns the execution runtime environment. Therefore, it is able to provide nearly transparent store and retrieve operations for objects in memory with transactional consistency. For better or for worse, it provides application programmers the illusion of an infinite amount of persistent, transactional RAM. Additionally, Gem Stone provides a query facility for stored objects and the ability to map objects to a relational database.
Gem Stone comes in two basic flavors, Gem Stone/S for Small Talk and Gem Stonej for Java Language. Gem Stone/S is now available as a free non-commercial download for AIX, HP-UX, Solaris, Windows and Linux from the gemstone site: www.gemstone.com 
For more information:
Gem Stone home page:
www.gemstone.com
Gem Stone FAQ from a Gem Builder-Smalltalk perspective:
www.ipass.net
Please correct me where I'm wrong below, gang. I'm trying to get a handle on Gem Stone's architecture and how people developed (develop?) applications using it. -- Robert Church
Gem Stone is a Smalltalk implementation with native object persistence.
Clients (most commonly running in client-oriented Smalltalk image like Visual Works) connect to the Gem Stone server and execute Smalltalk code in it. The clients also have semi-transparent access to persistent objects resident in the Gem Stone server. Business objects and associated logic reside in the Gem Stone server.
This differs from what is typically described as an Object Oriented Database. Gem Stone provides an object-oriented language that more than fills the role of Stored Procedures in RDBMS systems, while most other OODBs do not provide an in-database language.
Citation: The Gem Stone object database management system Communications of the ACM 34, 10 Paul Butterworth , Allen Otis , Jacob Stein -- October 1991 -- online at: portal.acm.org
See original on c2.com