Testability-explorer is a tool which analyzes java byte-codes and computes how difficult it will be to write unit-test. It attempts to help you quantitatively determine how hard your code is to test and, where to focus to make it more testable.
Test metric tool can be used:
As a learning tool which flags causes of hard to test code with detailed breakdown of reasons.
To identify hard to test hair-balls in legacy code.
As part of your code analysis-toolset.
As a tool which can be added into continuous integration that can enforce testable code.