BlockShopper Chicago Commercial site that has information on trends, foreclosures and individual sales for Cook County. Other cities are available as is Lake County, Illinois
Trade associations primarily serve thie members but they often have information available to non-members.
Counselors of Real Estate Institute of Real Estate Management
National Association of Realtors
Society of Industrial and Office Realtors
Depaul Institute for Housing Studies
MIT Center for Real Estate
Lincoln Institute of Land PolicyIncludes aggregate data on land value and property taxes.