The Rally .NET SDK by Joshua Stupplebeen interacts with the API, even though Rally Software does not maintain the kit actively. User guide provides links to access REST API and Web Services documentation. For support, ask a question at http://stackoverflow.com/questions/ask?tags=rally
In an agile world where the majority of application development is now iterative the testing of applications has never been more important. After all, the impact of single undiscovered error can now be easily compounded through multiple iterations of an application; making it that much harder and expensive to discover and fix later on.