Pure-PHP implementations of:

BigIntegers
RSA
SSH2
SFTP
X.509
Symmetric key encryption
    AES
    Rijndael
    Twofish
    Blowfish
    DES
    3DES
    RC4
    RC2

phpseclib is designed to be ultra-compatible. It works on PHP4+ (PHP4, assuming the use of PHP_Compat) and doesn't require any extensions. For purposes of speed, mcrypt is used if it's available as is gmp or bcmath (in that order), but they are not required.

SuperTuxKart is a 3D open-source arcade racer with a variety characters, tracks, and modes to play. Discover more...

VirusShare.com is a repository of malware samples to provide security researchers, incident responders, forensic analysts, and the morbidly curious access to samples of live malicious code.

This is software for producing a 2D animation.

It is based on the software "Toonz", which was developed by Digital Video S.p.A. in Italy, customized by Studio Ghibli, and has been used for creating its works for many years. Dwango launches this OpenToonz project, in cooperation with Digital Video and Studio Ghibli.
This software can be used by anyone free of charge, no matter whether the purpose of use is commercial or not.
This is available as an open source, so that users can modify its source code freely.
We aim to develop a new platform for connecting the academic research into frontline animation production.