I\'ve used the MIT Continuous testing plugin in the past, but it has long since passed out of date and is no longer compatible with anything approaching a modern release of Ecli
I also use Infinitest (and voted for one of its answers), but I wanted to add another approach, which relies on the build server. Whenever you want to implement something, create a branch in your VCS, do your changes, commit to your branch. If you have a build server configured, which runs unit tests on every checkin, your unit tests are then run on the build server without actually having polluted the trunk (or HEAD, whatever you call it) and without you waiting for the test run to finish.
I admit that this is not really continuous unit testing in the sense you asked the question, but for large projects or large test suites even a "normal" continuous test runner may slow you down way to much.
For small projects I also recommend Infinitest or CT Eclipse.