Version 7.6 is on Github under CC-non-Commercial: https://github.com/dybskiy/redactor-js

Even version 8.03 (august 20120)seems to be free (http://imperavi.com/webdownload/redactor/getold/), the new licenses got apparently introduced in 8.1.0, which adds a plugin system and some other stuff (http://imperavi.com/redactor/log/)