Apache Sling is almost painfully hip, in a way only a dedicated nerd could appreciate (or, ok, believe) — it provides a RESTful frontend to a Java Content Repository, and the whole thing is based on OSGi. Roughly, it gives you a content repository with customizable processing and presentation for different types of content, and the only ‘driver’ you need is a library that truly understands HTTP.
As part of evaluating it for the day job, I put together an s5 presentation with that other reST, and the result is Apache Sling Overview. I also dug into the codebase to figure out a bit more about Sling’s default POST processing servlet. I do hope I didn’t say too many materially false things.