8-13-14 This project is brand new and probably too late for the Hackaday Prize, but whatever...
It is based on an Electric Imp (www.electricimp.com) that very easily connects a sensor or switch to the Internet. This is the type of project where if you know how an Electric Imp works you immediately know how to do it, and if you are not a techy (like my tennis player buddies) it is MAGIC. I've explained what I'm doing and they have no idea how it might work but they do know how to check a web page (most of them, anyway). They are psyched and have started pestering me about getting it done.
I like this project because it has a high ratio of Utility to Effort, which is somewhat unusual in my experience.
So, technical details. Based on an Electric Imp with an April Development board (Amazon Prime, of course). Also has a toggle switch and a RGB LED and either a battery holder or a connector to a small power supply brick.
The plan is to put the RGB LED in one of two tennis ball lights I've found and ordered ($4 and $7 respectively). If this looks too dorky I'll just have the light on the box. The picture here is a terrible mock-up of what it might look like, but hopefully you get the idea.
The light will help the staff know they have the status set correctly and will let players know the status of the courts when they walk in the front door and want to know if the courts are open (again a constant source of (minor) annoyance to the staff).
I do a little web site development so that side of things is simple. I have a temporary web page now that I'm using. I registered www.TennisCourtsOpen.com and will use that eventually.
Still not sure how to power this. There probably is no problem using a small brick power supply but the best place to put this might not be near a wall outlet. Since the RGB LED is on all of the time it likely will take a fair amount of power. I won't know until I see how hard I have to drive the LED to make it visible inside the tennis ball light.
More to come...