This is a GREAT idea. I've got 4 kids and we put them in all sorts of camps/activities in the summer. Last summer they did tennis camp, surf camp, dance camp, baseball camp, art camp, and some others I can't recall. So, this is definitely something I'd use.

Now, as for the implementation, here's what frustrates me: You don't serve my city (San Luis Obispo, CA). Furthermore, you don't state anywhere on your home page "Serving San Francisco & Seattle. Put in your email and zip code and we'll notify you when we have your town." You should REALLY do that.

For this issue, you should also take a "backwards compatibility" approach (as Chris Dixon put it [1]). Getting more camps in new cities is hard–I get it. But, just about every city in America has a parks and recreation department and most of them publish their summer activities in a PDF guide. So, you get an intern and have him/her gather the URLs to every PDF available in California. Then Oregon. Then Washington and so on. I come to your site and put in my zip code–you say "Hey, we don't have anything in your city yet but here's your local activity guide" BAM ... backwards compatibility.

Marketplaces are hard. Keep up the good work.

[1] http://cdixon.org/2009/08/25/six-strategies-for-overcoming-c...