I've checked it out, and it seems like a pretty cool charity. Here's a little more background from their site:
If you will be or can be in New York in early November and are up for doing a marathon (and by a marathon, I mean one of the largest/most famous marathons in the world) you should look into partnering with the Fresh Air Fund, basically, they are your ticket in as the marathon itself is closed. If you can't make it to run the race, they would be happy to have your support in other ways too. If you are at all interested please email firstname.lastname@example.org or call (212) 897-8890.
If you need some more inspiration, here's a little Fresh Air history in pics:
and a little NYC marathon inspiration too.
If you can help out in anyway, even if it's just spreading the word, your contribution would be greatly appreciated.
(They need to have their runners registered by Sept. 30th.)