http://www.isminecraftdown.net
However, all this does is ping the server to see if the server responds. It's entirely possible for the server to respond to a ping, and yet web services be down. If you access minecraft.net right now, you get a 503 error, which is just that; so the java that redirects the game to connect you to the server is also 503'ing.
![Sad :(](./images/smilies/icon_e_sad.gif)