Google Web Toolkit

The Google Web Toolkit (GWT) is an open source framework for building browser-based applications that provide a rich user interface.

Learn more: http://code.google.com/webtoolkit

smartGWT

smartGWT is a comprehensive open source UI widget library for GWT-based applications.

Learn more: http://code.google.com/p/smartgwt

QuickFIX Engine

QuickFIX is an open source FIX engine, currently compatible with the FIX 4.0-4.4 specifications. It runs on Windows, Linux, FreeBSD, Solaris and Mac OS X. API's are available for C++, Java, .NET, Python and Ruby.

Learn more: http://quickfixengine.org

QuickFIX/J

QuickFIX/J s a 100% Java open source implementation of the popular C++ QuickFIX engine.

Learn more: http://quickjfixengine.org

QuantLib

The QuantLib project aims to provide a comprehensive software framework for quantitative finance. QuantLib is a open source library for modelling, trading, and risk management in real-life.

Learn more: http://quantlib.org

Esper

Esper is an open source solution that supports Event Stream (ES) and Complex Event Processing (CEP). Esper enables the rapid development of applications that process large volumes of incoming messages or events. Esper can filter and analyse events in various ways, and respond to conditions of interest in real-time.

Learn more: http://esper.codehaus.org

Marketcetera

Marketcetera is an open source trading platform that provides solutions for strategy automation, integrated market data, multi-destination FIX routing, broker neutrality and more.

Learn more: http://marketcetera.org