The Open RETS Client (ORC) is a command-line application written in Java, designed to retrieve Multiple Listing Service (MLS) related data from a remote/local Real Estate Transaction Standard (RETS) compatible server. The ORC is licensed under the GNU GPL
The source code is not available for download from the openretsclient.sourceforge.net website (as of June, 2006).
Download: Open RETS Client 0.8.1alpha Source Code [wi-fizzle.com]
Additional information and resources regarding RETS (RETS stands for the Real Estate Transaction Specification) is available from the official RETS website as well as the WikiShidia article dedicated to the Real Estate Transaction Specification.