Ning, a website that allows one to create their own social networking website (a meta site for those geeks reading this). It is my pick for the sleeper coolest thing I've played with lately. I peeked at this a year or so ago because I wanted to run a social network for my wife's sports league. Back then I thought it needed more bake time to be useful (putting it nicely). Well, while not perfect, it is quite nicely baked now. After playing with it for less than an hour my brain was teaming with cool ideas on how I can take advantage of this site.
First thing I did was create a social network for my extended family. Everyone posts pictures and vids and it's so simple an adult could do it. Next I got my wife to create that social network for her sports league. She used the business version so that she could use adsense with it and maybe make a few bucks :-). Next on my list is to create one for my kids pre-school. This thing is flat out addictive. It even has an api and gives pretty low level access to things like css. One can pay a little extra and have a custom domain name and remove all references to Ning.
Support is good do. I sent a suggestion that I wanted a calendar for my family on ning and they got back to me in under 24 hours that one was on the way. Hey maybe someday they will cluster with Terracotta ;-).
Anyway, when I played with this way back when I was a little sceptical but now I'm sold. Kudo's for the team at Ning for coming up with a cool idea and executing on it well.
update: One person pointed out to me that they didn't feel this entry was particularly developer focused and that it is more of a review of a website (not their is anything wrong with that). I see that point but I believe I am not reviewing a website but am reviewing a development tool to a certain extent. Ning is a meta-website. Anyway, dig in and tell me if I'm crazy.