BerkeleyDB
The Open Source license for Berkeley DB permits you to use the software at no charge under the condition that if you use Berkeley DB in an application you redistribute, the complete source code for your application must be available and freely redistributable under reasonable conditions. If you do not want to release the source code for your application, you may purchase a license from Sleepycat Software.
Dave McCusker has had some issues with them.
Sleepycat is also working on an XML-native db: Berkeley Db Xml
Here's an old email about performance of BerkeleyDB as a back-end to Zope's ZEO/ZODB (instead of the native data.fs file).
See SubVersion for a side note regarding running BerkeleyDB on MsWindows (Win95/98).
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