Inventory:  Actual inventory on the market. Current residential housing inventory from the Multiple Listing Service.  This can be obtained through the local Association of REALTORS. 

Prices: Median Housing Prices: also can be obtained via the Multi-list.  Other good sources cnn/money online http://money.cnn.com, and the US Cencus Bureau www.census.gov/.  Be sure not to compare from two different sources.  Each entity uses different criteria for measurements.

Rents: This is the most difficult to obtain and most important indicator for the purposes of tracking a market.  In Finding the Next Hot Market I discuss starting to identify markets while they are at the peak because that is when they are the easiest to identify.  This is also a good time to begin tracking rents.  Watching for stabilization and then an increase.  The most difficult issue if finding a reliable for the history of rents.  Commercial Apartment owners typically track this statistic, but individual investors that own 3 bedroom 2 bath homes are not as disciplined.

Sales: This is again actual Sales from the Multi-list data.  Watch closely this can be measured monthly in most major markets.  Take the current inventory and divide it by the current sales and you will have the absorption rate.  Less than 3 months is a strong market, greater than 9 months is a slow market. (21,000 Homes for sale, 7000 Homes sold in June = a 3 month supply of homes).  This statistic can and should be measured in different price ranges and by sub-market.

Foreclosures: There are two types (now three) of ways foreclosures are measured.  One is the Notice of Election and Demand this is the formal noticed filed in the court (therefore public record) to inform the homeowner that the bank will be foreclosing.  Second is the actual foreclosed property on the market. (the third is now the reports from the banks of current borrowers that are in some type of default buy an Election and Demand has not been filed).  It is important when gathering your information to understand that FOR THIS CHART WE ARE USING ACTUAL FORECLOSED PROPERTIES THAT ARE ON THE MARKET FOR SALE.  The other indicators are also important but due to the way investors and hedge funds purchase bulk properties some never hit the market and go directly into the rental pool.