A Concise Introduction to Logic
A logic textbook by Craig DeLancey of SUNY Oswego.
This site supports the proof system used in DeLancey's text.
Open Logic Project's proof editor and checker
An open-source proof checker developed as part of the Open Logic Project.
This site was developed from the Open Logic Project's proof checker by students and faculty at California State University, Monterey Bay (Jay Arellano, Mustafa Al Asadi, Glenn Bruns, Corey Hunter, Ben Lenz, and Gautam Tata).
The code for this site is available here and is licensed under the GNU General Purpose License (GPL) v3.