SOAP web services in erlang
Yaws (an Erlang Web Server) has a Soap module but Soap is not much used in the Erlang Community. Json is a better bet.
The Mochiweb kit that you can used to build your own servers (don't panic its easier than it seems) has good Json support.
Nothing to add to the above answer except that Mochiweb now has a new address: http://github.com/mochi/mochiweb