If you check out the comments on configure-your-ide-to-run-your-tests-automatically there are a slew of references to Infinitest which is a continuous testing tool, which has a very similar feature list to JUnitMax, which is also free.
I got the plugin up and running on a small - medium project (387 unit-tests). My project is laid out
Immediately I had a problem when trying out the plugin, it was running all my integration tests. After much rooting around it turns out you can add the special infinitest.filters file:
From readme.pdf of http://infinitest.googlecode.com/files/infinitest-4.0.3.zip
You may have tests in your classpath that you don't want Infinitest to run. These tests can be filtered out by creating a file in the working directory of your project named infinitest.filters. It should contain one regular expression per line. Any class names (not file names) that match any regular expressions in that file will not be run. For example: org\.myproject\.acceptance\..* .*\$.* will filter out all the classes in the com.myproject.acceptance package, and any inner classes (which always contain a $).
I added my filters and everything seems to work. Infinitest doesn't run all your tests, just a subset of them (it does some voodoo to look for dependencies), it tells you how many it ran, but it doesn't tell you which ones it ran... which is fine (I guess) but the anal part of me really wants more insight into what tests it's running. For example, I modified a class, Infinitest informed me that it ran 51 tests, but really which ones did it run... I just really want to know!
Also another interesting tidbit, from http://www.benrady.com/2009/04/comparing-infinitest-and-junitmax.html
The other function that I think a CT runner needs to do is test selection. If you're doing TDD, you're probably running a single test every time you make a change. If you're doing CI, you're running all your tests on a semi regular basis. Somewhere in the middle of these two approaches is a good balance of feedback quality vs speed. I think that's where CT tools should be focused. Infinitest uses dependency analysis to determine what tests need to be run for a given change.
Infinitest isn't particularly speedy, all my unit tests took .6 seconds to run, and the 51 Infinitest chose to run took 3 seconds. So maybe it's important for Infinitest to trim what it's running, but if your unit tests are fast to run than it's more of an Infinitest implementation detail vs a real time to run issue.
Anyway, I can't wait to start trying this tool as part of my workflow.