Zimbra uses Derby for offline storage!
When I was at the Web 2.0 Forum, there was a chat with a number of startups who got launched last year at the conference, and one of them was Zimbra. It was at this session that Zimbra announced their support for running the Zimbra client offline. I was impressed and was going to blog about it as a real-world example of the need for offline support.
Then I bumped into Stephen O'Grady's blog, and found out that they use Derby. Wow! Given that the Derby team has never heard a peep, that I know of, from the Zimbra team, this was quite a surprise (and I believe a sign of how robust and easy to use Derby is).
the comments to the Zimbra blog for some tantalizing details about how they did it and why they chose Derby.
Very interesting to see they're using Lucene for search and indexing. A number of folks in the Derby team believe there's an opportunity for tighter integration between Derby and Lucene if only we had time to work on it...