MapStudio OS is a Google open source project (LGPL license) that does what MapGuide Studio (an Autodesk products does, roughly). I thought the OS tool for this was going to be Web Studio from DM Solutions.
via a comment at SlashGeo
The National Association of Realtors (NAR), with approximately 1.3 million real estate broker and agent members nationwide;
Nationwide real estate brokerage firms, including RE/MAX and Keller Williams Realty;
Home builders, such as Pulte Homes and Ryland Group;
The National Association of Home Builders (NAHB);
Enterprise computer software providers to the real estate industry, including Fidelity National
Real Estate Solutions, First American Corporation, and others;
MOVE, Inc., the company that operates flagship real estate websites such as Realtor.com, MOVE.com, HomeBuilder.com, RentNet.com, and SeniorHousingNet.com; and
More than 25 other defendants representing nationwide classes of claimed infringers: real estate brokers, agents, multiple listing services, new home builders, and rental property owners/managers.