I’ve recently started trying NCrunch. It’s commercial software with a 60-day trial which helps you make sure you are getting good code coverage and you’re not breaking tests.
What it does is fairly straightforward. It’s a test runner, like VS’s Test Explorer, except that it automatically builds your tests and runs them, optionally on many threads, in the background. It then reports on the health of those tests, so if you start breaking tests, you know about it quickly.
It also shows you, in the VS document editor, which lines are covered by unit tests. So you can see where you need to write more tests.
There’s a nice writeup on Olav Nybø‘s blog.