PET is a proxy server that works for HTTP (with caching), SSL, and FTP. PET supports extended ICP services (enhanced algorithms) and Cache Digests, both of which are compatible with Squid. It also includes an easy-to-use Web-based configuration interface for nearly all configuration parameters and statistics.
Proxy is a C application that when run on a multi-homed host will forward all packets from source, to destination. Where source might be a system on the internet, and destination might be a box on a private network behind a multi-homed linux machine. It's also good for setting up sort of a 'bounce back' connection for poeple trying to attack you. Just run the proxy and point the outgoing connection to your attackers system, and they will end up attacking their own machine.
RabbIt is a mutating, caching Web proxy used to speed up surfing over slow links like modems. It does this by removing advertising and background images and scaling down images to low quality JPEGs. RabbIT is written in Java and should be able to run on any platform. It does depend upon an image converter if image scaling is on. The recommended image converter is "convert" from the ImageMagick package.
rproxy is an extension to HTTP which allows transferring of differences between a web server and client, using an algorithm similar to rsync's. This works particularly well for dynamically-generated sites, where the pages are often similar but not the same on each visit. Transferring just the difference reduces network traffic and therefore loading time.