Download of ACL2s 4.4.2 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software. This free Mac app is a product of Georgia Tech Research Corporation and Northeastern University. The application is categorized as Education Tools.
From the developer:
ACL2 is a powerful system for integrated modeling, simulation, and inductive reasoning. Under expert control, it has been used to verify some of the most complex theorems to have undergone mechanical verification. In addition to its maturity and stability, these qualifications make it a good platform for learning about industrial-strength theorem proving.
You may want to check out more software for Mac, such as Integrated Data Viewer Installer, UN-SCAN-IT Demo or ScienceMatrix, which might be similar to ACL2s.