From the librets website:
libRETS is RETS client library written in C++ that allows rapid development of RETS client applications. By saving the developer from dealing with the RETS protocol details, the application writer can concentrate on their application, saving them time and money in the process. It is released under a BSD-style license.
* Provides access to login, logout, search, and get object.
* Can connect to RETS 1.0, 1.5, and 1.7 compliant servers.
* Can be used on many platforms, including Visual C++ on Windows, and gcc on just about any flavor of Unix.
* Native bindings for C#, VB.Net, Python, and Ruby.
These details are provided for information only. No information here is legal advice and should not be used as such.