MetaPRL logical programming environment  
   The next generation of the NuPrl proof development system. The main new features of MetaPRL include: 1) Modularity. Programs and logics are developed as modules that define computational, heuristic, and mathematical properties. 2) Speed. MetaPRL is more t
   http://cvs.metaprl.org:12000/metaprl/

The LEGO Proof Assistant  
   A powerful tool for interactive proof development in the natural deduction style. It supports refinement proof as a basic operation. The system design emphasizes removing the more tedious aspects of interactive proofs.
   http://www.dcs.ed.ac.uk/home/lego/

Alfa  
   A successor to the proof editor Alf with a graphical user interface, being developed at the Programming Logic Group at Chalmers. Available for download.
   http://www.cs.chalmers.se/~hallgren/Alfa/

NuPrl Proof Development System  
   A powerful tactic-based proof assistant, developed over the last 15 years at Cornell University. IFeatures include: very expressive logical language based on Martin-Lof type theory, extensive library of formal mathematics and automata theory, possibility
   http://www.cs.cornell.edu/Info/Projects/NuPrl/

Proof General  
   Emacs based generic interface for theorem provers.
   http://zermelo.dcs.ed.ac.uk/~proofgen/

Kumo  
   A web-based proof assistant. It assists with proofs in first order hidden logic, using OBJ3 as a reduction engine. The most important inference rules in first order logic and hidden equational logic are implemented, including induction and coinduction, ge
   http://www.cs.ucsd.edu/groups/tatami/kumo/

Af2 Proof Assistant  
   A type system based on second order intuitionistic logic.
   http://www.lama.univ-savoie.fr/sitelama/Membres/pages_web//RAFFALLI/af2.html

Isabelle  
   Homepage of the theorem prover environment developed by Larry Paulson at Cambridge University and Tobias Kipkow at TU Munich.
   http://isabelle.informatik.tu-muenchen.de/

The Practice of Logical Frameworks  
   Comprehensive survey article by Frank Pfenning (1996) discussing the history of work on logical frameworks and their current open problems.
   http://citeseer.nj.nec.com/pfenning96practice.html

The HOL Theorem Proving System  
   The system documented originated at the Laboratory for Applied Logic of Brigham Young University and features higher-order, classical, natural deduction with tactics.
   http://hol.sourceforge.net/

Yarrow  
   A proof-assistant for Pure Type Systems (PTSs), representing different logics and programming languages. A basic knowledge of Pure Type Systems and the Curry-Howard-de Bruijn isomorphism is required. (This isomorphism says how you can interpret types as p
   http://www.cs.ru.nl/~janz/yarrow/