Introducing the incredibly named XsltTester. It's pretty basic, and I'm sure that there are existing free tools that do the same thing. However, this took just 30 minutes or so and it was fun to put together.
Here is a link to the project on BitBucket: https://bitbucket.org/lee_wallen/xslttester
Here are a few views of the app:
New icons and buttons.
Example with XML and invalid XSL.
Example with XML and fixed XSL but wrong output.
Example with XML, fixed XSL and fixed output.