The Marketcetera trading platform is an open source (GPL license) written in Java, using other open source Java resources like ActiveMQ and Spring. The Marketcetera platform includes a trading GUI that is based on the Eclipse framework, a trading system that can run quantitative modules, a database interface, order processing and much more.
Marketcetera has a Confluence based Wiki that documents the software. To read the Wiki or download the software you need to register and get an account.
The Marketcetera software base looks quite large, so there is undoubtably a serious learning curve. However, it would take at least nine-months to come up with a basic trading system that only supported part of what Marketcetera supports.
I first learned about Marketcetera from a post in the New York Times Bits blog. Marketcetera is divided into two entities: the Marketcetera company and the Marketcetera open source project.