Wikipedia
YXORP
YXORP is a Reverse proxy for the HTTP protocol, written by Sytse van Slooten. The main focus for YXORP is checking the validity of the traffic, i.e. what would be called ' application level firewalling' or 'secure reverse proxy' in commercial products. It can check, validate, and change just about anything about an HTTP request or response, can load balance, and all other normal reversed proxy functions. YXORP aims to conform to RFC 2616, RFC 2518, RFC 2109 and other applicable standards.
YXORP is free software released under the GNU GPL.