Node-pixelserv is a project mainly written in JavaScript, it's free.
Pixelserv implementation in Node.js
I wanted to play with Node.js and to be honest - I thought this would be harder to do ;)
From the Pixelserv website:
Pixelserv is a super minimal webserver, it's one and only purpose is serving a 1x1 pixel transparent gif file. Using some creative firewalling (netfilter/iptables) rules you can redirect some webrequests (advertisements, for example) to pixelserv.