There are 'quite a few' Ajax frameworks in nearly the same way that there are 'quite a few' stars in the sky, however not many that do the same sort of thing as DWR. From what I've seen the biggest competitor DWR has is JSON-RPC, which has just released 1.0, so congratulations. See also the changelog and the downloads.
Mike Milinkovich mailed me the other day with a heads-up on RAP (Rich AJAX Platform), which seams to be a port of the Eclipse RCP platform to Ajax.
"The RAP project aims to enable developers to build rich, AJAX-enabled Web applications by using the Eclipse development model, plug-ins and a Java-only API."
There is a demo of it in action here.