MinGW: A collection of freely available and freely distributable Windows specific header files and import libraries combined with GNU toolsets that allow one to produce native Windows programs that do not rely on any 3rd-party C runtime DLLs...MinGW uses the Microsoft runtime libraries, distributed with the Windows operating system.
GDB, the GNU Project debugger, allows you to see what is going on `inside' another program while it executes -- or what another program was doing at the moment it crashed.
GNU DDD is a graphical front-end for command-line debuggers such as GDB, DBX, WDB, Ladebug, JDB, XDB, the Perl debugger, the bash debugger, or the Python debugger. Besides ``usual'' front-end features such as viewing source texts, DDD has become famous through its interactive graphical data display, where data structures are displayed as graphs.
ProofPower is a suite of tools supporting specification and proof in Higher Order Logic (HOL) and in the Z notation...All the ProofPower packages except PPDaz are free, open-source, software made available under the terms of the GNU General Public License. (PPDaz is Ada stuff)
Jape is an interactive tool designed to help with learning, teaching and using formal reasoning. It takes a description of a logic as a system of inference rules, and supports the development of proofs in that logic.