I've released a latest version of a simple OAuth client I developed. You can find version 1.1 of the jar here. Also the site for the project is here, and finally the source is here. I should have some documentation on basic usage up later.
So far the library only supports the HMAC-SHA1 method of signing, and is using query parameters to send the OAuth information. Soon I'll add support for using headers and POST data to send the OAuth information.