ACL2 is a mathematical logic, programming language, and mechanical theorem prover based on the applicative subset of Common Lisp. It is an "industrial-strength" version of the NQTHM or Boyer/Moore theorem prover, and has been used for the formal verification of commercial microprocessors, the Java Virtual Machine, interesting algorithms, and so forth.
MpNT is a multi-precision number theory library that provides a base for building cryptographic applications. It may also be used in any other domain where efficient large number computations are required. The library supports integer, modular, and floating point arithmetic with practically unlimited precision. It is both speed efficient and highly portable without disregarding code structure and clarity.
Tutorial Environment for Cryptographic Protocols (TECP) is a tutorial and visualization environment for public key cryptography. It is intended for students of cryptography courses and can be used to improve understanding of modular arithmetic-based public key algorithms, and for lecturers who want to add some life to boring blackboard-and-chalk presentations about the topic.
The OpenAI site is centered around an Open Source project and community involving artificial intelligence. The project itself is the creation of a set of tools that are considered to be models of human intelligence or biomimicry. These tools are intended to be integrated into applications or used stand alone for research.
distributed.net is a loosely knit group of computer users from all of the world that is taking up challenges requiring lots of computing power (most notably the RC5, DES, and OGR cracking contests). It is simple to participate in the challenges by downloading and running their client software (which uses idle CPU time to complete its tasks).