I found CouchDB referenced from one of the posts about Amazon's SimpleDB since they are both apparently written in Erlang.
It's interesting that people are exploring some alternatives to relational databases.
It's also interesting that people are implementing "real" products in alternative languages like Erlang.
I recently picked up Programming Erlang but I haven't read it yet.
One thing that caught my eye looking through the CouchDB web site was a brief note that they compact the database while it's running, by copying to a new database. Currently Suneido requires you to occasionally shut down the database in order to compact it. I had always thought about "on line" compaction in terms of doing it "in place", but that gets tricky due to updating indexes to point to new locations. But if you build a new copy you don't have that problem. You could copy the bulk of the database in a single read-only transaction (like the current on-line backup does) and then pause activity briefly to get any updates done during the copy, and then switch over to the new database. Hmmm... actually doesn't sound too bad. (famous last words!)