BCS Tester Man is a BCS Test Client that makes it easier for SharePoint BCS Model Developers to test Models and External Content Types. You'll no longer have to create and configure Business Data Web Parts or External Lists. It's developed in C#. UI is WPF with MVVM.
Having worked with BCS Models and External Content Types for a while one of the biggest pains is if a model deploys successfully it doesn't always mean that it is working. You have to open the page, edit the a bcs web part and then check it. If it's not working then you still need to go to the logs to find out what exactly went wrong. The Idea of BCSTesterMan is to just allow you to deploy your model, open up BCS Tester Man, select the method and just check to see if some data is returned. The benefit of having it in a client is that you can test all methods, some like the IdEnumerator are difficult to test normally.