A HTML page that computes an OAuth 1.0 signature, showing all the workings so you can debug your own implementation.
http://7digital.github.com/oauth-reference-page
It supports POSTs in the form of application/x-www-form-urlencoded, as well as POSTs of application/xml and application/json using the Authorization header - just put the XML or JSON into the 'body' field and set the 'body encoding' accordingly.
If you're using POST or PUT, it's best to copy the curl command output into a terminal window