XUProxy is an extensible multi-protocol proxy based on the Twisted framework. It supports multiple protocol plugins (currently only HTTP), and multiple "filter" plugins for things like logging, caching, and Proxomitron-compatible ad filtering.
Webtunnel is an HTTP tunnel with some unique features: it runs in the context of a web server; it uses simple requests/responses for proxy traversal; and it is multi-threaded to allow many parallel connections to many destinations simultaneously.