More info about sitemaps on http://sitemaps.org.

Note.
OSG is still in beta, because we need a wide testing, but we use it constantly to generate the sitemaps of our sites.
Are you interested in a Java porting of this software? Contact us.
 

Why we used db4o.
We needed a fast and simple way to store all the URLs that OSG crawls.
To achieve this result, db4o was the best solution: it's fast, we added it to our code in minutes and we didn't need to design a database structure and write a lot objects-to/from-tables functions.