OmniBase is a portable (Dolphin, VAST, VW and now Squeak) Object Database.
It has everything you need (good concurrency support, large collections,
etcetera) and has been in use at many production sites for quite some time.
The version distributed here is a version without garbage collection, which
means that the database will keep growing. If you want to use the database
for commercial projects, you need to obtain development licenses, which include
the garbage collector - see http://www.gorisek.com/ for more details.
Requirements on Windows:
Requirements on Unix or Mac OS X: