We are using Groovy to test our RestFUL API, using a series of helper functions to build the xml put/post/gets and then a series of tests on the nodes of the XML to check that the data is manipulated correctly.
We use Poster (for Firefox, Chrome seems to be lacking a similar tool) for hand testing single areas, or simply to poll the API at times when we need to create further tests, or check the status of things.