Go See: swspookshare.sourceforge.net
SpookShare is a information advertizing/searching protocol which can be used for file sharing. It works by posting messages over HTTP. A SpookShare node (also known as a SpookShare server) runs as a CGI on an HTTP server. The protocol is very flexible, and is fairly easy to implement. For more info, read the SWSS readme file. I encourage other people to write their own implementations of the SpookShare protocol, since mine is relatively crummy right now (as far as UI is concerned)