There have been fewer and fewer new Google apps as the company expands into every conceivable web and mobile market, but they still manage to surprise us every now and then. The latest Google app is called Who's Down, and it's a strangely specific social tool. Basically it's an all-purpose "available" button. Slide the toggle from on to off, and anyone you've connected with can tell that you're available for... whatever. You can select specific activities you'd like to take part in, see which friends are also "down," and chat with them in an integrated message service.

But don't rush to download it. At the moment it's in a closed beta, and you need an email invitation to sign in. Oddly, Google also asks for a school name when requesting an invitation, suggesting that the development team is currently targeting the app towards college students. That makes sense - students tend to have more free time than the rest of us, and they also tend to be on or around campus for large swaths of time. Even so, it's a bit of a bummer that most of us old-timers won't be able to use the app, at least for the moment.

It's also strange that this is a stand-alone app. Who's Down seems like a feature that might otherwise be integrated with Google Maps or Google+. Who knows, it might still be - we'll see if this beta-style app turns into something lasting.