Streeteasy

StreetEasy is excellent as a free real estate listing service. I found my current apartment on the site more than 5 years ago, and have recently started looking again. The site allows users to sort by price, neighborhood, and various amenities like elevators, doormen, etc. The vast majority of postings include photos, specific apartment numbers, and a map to determine cross streets (which is important in NYC). The site also discloses whether the unit is a condo, coop, or other type of building and if there is a brokers fee associated with the application.

Although there are some quality control measures around the postings, the sources of the postings can be a bit of a mixed bag. Most listings are for legitimately available properties, but some inevitably end up being unavailable units posted by real estate companies trying to drum up clients. (In NYC, any listing that's more than a few days old is likely already rented). However, I do get a much higher rate of responses from inquiries that I make via StreetEasy than from the actual realtor sites; sometimes multiple agents respond to a single inquiry.

All in all, StreetEasy is a great resource for assessing the rental market. You might even snag an actual property.