For me, it might be Dropbox. I keep thinking of new uses for it.
For a guy with a huge number of computers (I don't even want to count them), it's not only a lifesaver but an idea factory. I've already built utilities on it. The basis: polling a folder is incredibly low-cost. You can do a lot of it without impacting the performance of your machine. That was true in 2002 when we made Radio do upstreaming. It's even more true today.
Because Dropbox wires together folders on any machine you link into it, it's a very simple content distributor. You can have 18 computers looking for something, when one finds it, they all find out and get the thing. It could be large or small.
Like all cool things, it's fairly obvious, and has probably been done many times before. But they put it together now and it works and is trivial to set up. I keep thinking of things to use it for. All of which makes it very cool. Unless I'm missing something, it's my CSOTD.
Update: There's a thread on this topic on Ycombinator.
http://bit.ly/4C5J7U
