Is there any open-source implementation of a web-based trading platform? You know... something like MtGox, but open-source.
Every exchange seems to have rolled their own. Bitcoin-Central.net had at one time shared their code. Intersango too, before they rewrote the exchange that they use today.
Others, such as BitMe.com, wrote their own but never reached a sufficient volume to justify the effort. Developing an exchange that is secure is an expensive endeavor.