Use your browser
- RESTClient - allows you to generate REST requests from within the Firebug browser and view the responses
- JSONView - allows viewing of JSON content in Firefox
- Tools->Developer Tools - comes with Chrome and does mostly the same as firebug
- REST Console - allows you to generate REST requests
- JSONView - allows viewing of JSON content in Chrome
There are many other tools and options to help, but the above are a good selection to help get you started.
From a Java application
There are a lot of good Java libraries available to assist, some examples include:
- Apache HTTP Client - If you want to build your own - now being moved to Apache HTTP Components
- Jersey - Both Client and Server support is offered
- Abdera - an Atom reader but is still useful for HTTP access alone when using JSON as it manages authentication quite well
Regardless of the technology you use, it is useful to have a debugging proxy such as Fiddler to help you look at the requests and responses you are sending/receiving.