<p>What is the best, preferably free/open source tool for auto-generating Java unit-tests? I know, the unit-tests cannot really serve the same purpose as normal TDD Unit-Tests which document and drive the design of the system. However auto-generated unit-tests can be useful if you have a huge legacy codebase and want to know whether the changes you are required to make will have unwanted, obscure side-effects.</p>

To be honest, I probably wouldn't do this.  Unit tests are isolated and you won't actually know if you have "unwanted, obscure side-effects" because everything is walled off from the other things that cause the side effects.  As a result, you need integration or system testing and
that
is not something you can automate.
Build a few high-level, end-to-end system tests which give you a degree of confidence and then use coverage testing to find out what you've missed,  The downside is that when bugs crop up, it will be harder to point to their exact cause, but the upside is that you'll be far more likely to see the bugs.
Once you find bugs, write unit tests just for them.  As you move forward, you can use TDD for the bits you want to refactor.
I know this probably wasn't the answer you want to hear, but I've been testing for many, many years and this is a solid approach (though I would hardly call it the only approach :)