Only what's already in Asterisk itself. You might want to look at one of the traditional H323 gateways (Multitech, Vega at al). Other than that I think you may be out of luck.
On the plus side, we are very close to bringing out a new product of our own, just to help you link up those "big buck" switches. It will be PRI/SIP based for the most part but we will support BRI and analogue as well.
It looks very like it will be called Vgate and we'll be looking for Beta testers in the next few weeks So get your name in the Hat if you have access to one or more traditional TDM switches and you would like to hook 'em together to provide desk-to-desk, conferencing, unified Mail and all of that good stuff to these lumbering old clunkers.
Best
S