I got some things quite wrong, so here is a correction of my last
statement (thanks Neil B. for making me look deeper and learn):

Correction:  The Sleepycat license _IS_ free _and_ GPL compatible:
http://www.fsf.org/licenses/license-list.html

But you _cannot_ (at all) use it together with closed source products:


 It's not quite right to say that "under the Sleepycat license" they
 can ship closed source. They can't do that at all under the Sleepycat
 license -- it's effectively the GPL. If they want to ship closed
 source, they need to pay us for a different license. That license
 looks just like all the other agreements that vendors sign with each
 other. The "dual licenses" are the Sleepycat license and a separate
 license agreement for proprietary use and redistribution.