Sling

Have been checking Sling recently. Basically yet another Java web framework, but this one has a rather interesting approach - it is content oriented. It is based on OSGi, JCR, REST, server side scripting, etc. The recently popular principle convention over configuration is heavily used.
Found this brief intro to Sling.
Also this cheat sheet gives a condensed overview over the Sling way.

Although both projects Sling and Jackrabbit are hosted at Apache, they are led by a company called Day.
It is interesting that Roy T Fielding is a chief scientists at Day.