ACL2 is distributed on the Web without fee.
There is a License agreement, which is available via the ACL2 Home Page link below.
ACL2 currently runs on Unix, Linux, Windows, and Macintosh OS X operating systems.
It can be built in any of the following Common Lisps:
* Allegro Common Lisp, * CCL (formerly OpenMCL) * CLISP, * CMU Common Lisp, * GCL (Gnu Common Lisp), * LispWorks, and * SBCL (Steel Bank Common Lisp)