Prime95

Prime95 is the freeware application written by George Woltman that is used by GIMPS, a distributed computing project dedicated to finding new Mersenne prime numbers. More specifically, Prime95 refers to the Windows and Mac OS X versions of the software.

MPrime is the Linux command-line interface version of Prime95, to be run in a text terminal or in a terminal emulator window as a remote shell client. It is identical to Prime95 in functionality, except it lacks a graphical user interface.

Although most of the GIMPS software's source code is publicly available, it is technically not free software as users must abide by the project's distribution terms if the software is used to discover a prime number with at least 100,000,000 decimal digits and wins the $150,000 bounty offered by the EFF. As such, a user who uses Prime95 to discover a qualifying prime number would not be able to claim the prize directly ($50,000 will go to the person who finds the prime number). A free software package would not have this restriction.