Slashdot is reporing on Microsoft's promise to release a new ASP.NET AJAX framework that will add existing functionality to the Callback features already present. Scott Guthrie also talks about it.

So far here are ASP.NET frameworks I've found:

• Microsoft's Atlas (news)
  
• Ajax.NET
• CodeProject ASP.NET/AJAX
  
• My port of ASP.NET 2.0 Callbacks to ASP.NET 1.x  (not too useful since it's just the basic callback)
  

And some non-ASP.NET specific frameworks:

• Sarissa - http://sarissa.sourceforge.net/doc/
• Prototype - http://prototype.conio.net/
• Dojo - http://dojotoolkit.org/
  
• SAJAX - http://absinth.modernmethod.com/sajax/
• DWR - http://www.getahead.ltd.uk/dwr/
• JSON-RPC-Java - http://oss.metaparadigm.com/jsonrpc/

Some other cool call back releted stuff here:

• Script.aculo.us