Exchange is a project mainly written in RUBY and JAVASCRIPT, it's free.
bitcoin exchange
Open Source Bitcoin Exchange
http://www.bitcoin.org
Created as a response to closed source exchanges like: Mt Gox: http://www.mtgox.com Tradehill: http://www.tradehill.com Ruxum: http://www.ruxum.com
Please fork/commit/push :)
To run, git clone the source, then cd in directory 'bundle install' and 'thin start'
I have code for interacting with dwolla and bitcoin daemon, but they're under construction