Infinitest is an Eclipse plugin that runs the JUnit tests instantly when you save a Java file. Check out the demo above. It smartly runs only the test affected by the file you just saved. This is cool. You can immediately know if you broke something as soon as you save your code. Of course, your test cases should be comprehensive. Improving Works developed this plugin and has released it as a open source project.
Infinitest is a continuous test runner for Java.