This is the type of site that shows why maps mashups are so popular; when the right data is displayed on a map it just comes to life. Rentometer will let you know if you are paying too much, too little, or just the right amount of rent for your unit type and area. It's incredibly cleran and straightforward interface makes it a breeze to use. It utilizes Yahoo! maps and "data from publicly available sources on the internet and in print". The site comes from iiProperty, a national property management firm -- this will definitely raise their profile. I would like it if they made their data sources clearer, but the data doesn't seem to be outrageous for the areas that I checked.
[via Providence Geeks]