Dante is a free implementation of the proxy protocols SOCKS version 4 and SOCKS version 5 (RFC 1928). It can be used as a firewall between networks, controlling outgoing traffic. The package consists of two parts: a socks server and a proxy client that supports socks, HTTP proxies, and UPnP. RFC 1961 (GSSAPI) is supported in both the client and the server. Commercial support is available.
PHP-Proxy is a PHP script for taking Web pages from one server, so that your main server is protected if the Web server is overloaded from things like DOS attacks. It is also very useful if you have a machine behind a firewall that you want to be your server for Web pages, provided that the machine running the proxy has port 80 open. Finally, there is encryption to defeat filters on specific words if need be.
EXProxy is a simple HTTP/HTTPS proxy server packaged as a Java library and designed to listen to HTTP requests and responses going through it. It has also the ability to modify requests and responses on the fly. For HTTP over SSL, it uses the "man in the middle" principle, and so will generate warnings about certificates in your Web browser.
pacparser is a library to parse proxy auto-config (PAC) files. Proxy auto-config files are a vastly used proxy configuration method these days. Web browsers can use a PAC file to determine which proxy server to use or whether to go direct for a given URL. The idea behind pacparser is to make it easy to add PAC file parsing capability to any program (C and Python are supported right now). It comes as a shared C library and a Python module that can be used to make any C or Python program PAC scripts intelligent. Some very useful targets could be popular Web software like wget, curl, and python-urllib.