A pocket-friendly version of PadMapper means that next time you're walking around in a neighborhood that looks promising, you can just pull out the app and let PadMapper use your location to find available rentals nearby. (You can also search for a specific location if you don't want to view nearby listings.) Apart from the portability, it works almost exactly like the web version. You can filter by rent, number of bedrooms and bathrooms, view full listings and pictures, and save favorites.
PadMapper is a free download from the iTunes App Store, and I don't think I'd start another apartment search without it.
PadMapper on the iPhone [TopHat Monkey Software]
