QuickFIX is an open source implementation of FIX protocol, currently compatible with the FIX 4.0-5.0 specifications.
The Financial Information Exchange (FIX) Protocol is a message standard developed to facilitate the electronic exchange of information related to securities transactions. It is intended for use between trading partners wishing to automate communications.
That being said, the purpose of FIX is to make communication between financial vendors trivial, leaving them to focus on their core business. The FIX organization also specifically cites that openness has been key to their success. QuickFIX intends on expanding the openness of FIX into not just the specification of, but also the implementation of the protocol.
So what is QuickFIX? Simple. QuickFIX is a free and open source implementation of the FIX protocol. QuickFIX is not crippled, all of the source is open for all supported versions of FIX. There is no upgrade to a commercial version.