QuickFIX Engine:

QuickFIX is a full-featured open source FIX engine, currently compatible with the FIX 4.0-4.4 spec. It runs on Windows, Linux, Solaris, FreeBSD and Mac OS X. API's are available for C++, Java, .NET, Python and Ruby. [http://quickfixengine.org/]

QuickFIX/J:

QuickFIX/J is a full featured messaging engine for the FIX protocol. It is a 100% Java open source implementation of the popular C++ QuickFIX engine. [http://www.quickfixj.org/]

QuantLIB:

The QuantLib project is aimed at providing a comprehensive software framework for quantitative finance. QuantLib is a free/open-source library for modeling, trading, and risk management in real-life. [http://quantlib.org/]