Where is the open source project for the WMD editor used on this site?
Since Google Code archive all repositories, pagedown now lives at GitHub https://github.com/balpha/pagedown.
As of August 2011, it now lives under http://code.google.com/p/pagedown/. This version is identical to the version running on the Stack Exchange sites, except for a few Stack Exchange-specific additions (e.g spoilers) that are not included.
It is a completely refactored version; the old one at http://code.google.com/p/wmd-new/ is no longer maintained.