Its called Magma. Its implemented in Squeak Smalltalk and runs fine on Pharo and is open source. So far I've enjoyed using it, but I haven't had to scale it yet. It does support replication with automatic fail-over though.

Edit: Some basic docs: http://wiki.squeak.org/squeak/2665 The rest of the docs are just "read the code/comments" which actually works in Smalltalk. I've found "read the code" never works well in any other language due to poor code navigation in file-based PLs

Edit Edit: Sorry, didn't read your comment entirely. Its a pure OO database. These are pretty common in Smalltalk as it has good built-in serialization/marshaling routines, so most OO dbs are just a layer on top of the built-in stuff.