Is it free? Yes, in two different ways: one, it's no charge to get it (and this does apply to 4.6,3 and 5.7.0 (the latest)).
The other way it's free, is it's free software, which means the license allows you to get it, change it, document your changes and release it. You can even fork it, if you must.
I share Gustaf's question, and rephrase it as: do you want to try the latest released version? it's openacs-5.7.0, it works with postgres-8.3.x and aolserver-4.5.1.
