A PowerShell library for automating tests. You can use this library to test your scripts, but it is really designed to test things external to PowerShell: your Active Directory configuration, your SharePoint installation, etc.
This project was initially released for PowerShell v1.0 and has since been revised so that it works for PowerShell v2.0 (CTP and above) as well. I've not run the full suite of tests on all OS/version combinations though, so please log an issue if you find something wonky.
I built this so that it would be easier to employ PowerShell as a means of automating tests. I believe the language has features that fundamentally make it a wise choice for automating tests, in particular, when the targets of the tests happens to publish cmdlets. So while you can use it to test your scripts, it really is intended to provide you with the means of writing tests to support the work that you are already doing, whether that is writing scripts, configuring commercial-off-the-shelf products, or developing an application.
Note: The decision not to use cmdlets at all in this solution was to avoid the need to install anything on the machines running the target of the tests. This is also why relying on an existing .NET test framework (such as NUnit) wasn't considered the optimal solution.