DC Proof Online
Download free proof writing software for high school, college and university-level mathetmatics
http://www.dcproof.com
NotCH Web Page
Science, Math, Logic and Foundations, Set Theory - Burke, Dennis. Miami University of Ohio.
http://notch.mathstat.muohio.edu/
Guglielmi, Alessio
Science, Math, Logic and Foundations, Proof Theory - Guglielmi, Alessio. Technical University of Dresden, Germany.
http://alessio.guglielmi.name/
KR, Inc.
Science, Math, Logic and Foundations, Organizations - KR, Inc.. Principles of Knowledge Representation and Reasoning, Incorporated.
http://www.kr.org/
ASL
Science, Math, Logic and Foundations, Organizations - ASL. Association for Symbolic Logic.
https://www.aslonline.org/
Chu Spaces
Science, Math, Logic and Foundations, Nonstandard Logics and Extensions - Chu Spaces. Site created by Vaughan Pratt.
http://chu.stanford.edu/
Institute for Logic, Language and Computation.
The Institute for Logic, Language and Computation (ILLC) is a research institute of the University of Amsterdam. ILLC's central research area is the study of fundamental principles of encoding, transmission and comprehension of information. Research at ILLC is interdisciplinary, and aims at bringing together insights from various disciplines concerned with information and information processing, such as logic, mathematics, computer science, linguistics, cognitive science, artificial intelligence and philosophy.
https://www.illc.uva.nl/