SPARQL
A SQL-like query language for RDF. Very handy for AJAX applications as it supports HTTP and JSON.An OSS Java implementation using Jena and packaged with Jetty:
http://sparql.org
From the Opera web browser folks: http://my.opera.com/community/sparql/
JSON and graphs
The JSON result format is only for result sets (tablular select queries). Dan Connolly mentions the desire for handling more general results.