If you're looking in the outer boroughs it may be worth looking on neighborhood blogs. For instance, if you're looking in Astoria, a good place to try might be the classifieds and apartments forum on astorians.com .
May neighborhoods have online forums and blogs that cater to those already living in the neighborhood - this might be a good inside way of finding something.
google has a blog search here Google Blog Search
just type in the name of the neighborhood, such as "park slope" and at the top of the search results you will see the main blogs that focus on park slope.