Stable versions are the versions that you find at the top of the downloads page on ispconfig.org:
ISPConfig 3 is the next generation of the ISPConfig control panel, rewritten from scratch to support virtual users and multiple servers. ISPConfig 3 can not be used to Update a ISPConfig 2 installation at the moment.
Download ISPConfig 188.8.131.52 [Download] Info
System Requirements for the beta version: Debian or Ubuntu. Experimental Support for Fedora, Centos and OpenSuSE. Installation instructions can be found inside the tar.gz file.
All other versions are testing versions are either betas or svn versions.