I'm not sure what's cooler about RingMeMaybe: the name or what it does.
It's a toss-up. RingMeMaybe generates disposable phone numbers right on your iPhone, the idea being to protect your privacy while still making yourself available by phone.
Let's say you're selling something on Craigslist, which, as we all know, can. Ideally, you'd rather not give out your real home or mobile number. Same goes for the guy you just met at the bar.
A temporary number allows for phone contact without making your real number public.
You can register the app via Facebook or an e-mail address. Once that's done, all it takes is one tap and RingMeMaybe generates a number (though not necessarily with a local exchange, alas), then lets you enter a tag (such as "Craigslist," "dating," or the like). It's good for seven days, though you can delete it sooner if you're done with it. There's also a pause option that can temporarily disable the number. And the app's Share button lets you distribute the number via e-mail, Facebook, or Twitter, or copy it directly to your clipboard.
Calls to your number actually come through the app, which keeps a list of received calls so you can review them later (and, if necessary, share them with the police, ha-ha).
RingMeMaybe runs on credits: 10 per phone number generated, with 20 provided free of charge at the start. If you decide you need to keep a disposable number for longer than seven days, another 10 credits extends it for another week. It costs 99 cents (via in-app purchase) for each "pack" of 10, with slight discounts at the 100-credit level. Why the fees? In part because there's some voice-over-IP action at work here.
Granted, there are a few Web-based services out there that will create disposable phone numbers for free, and you can accomplish much the same thing with a free Google Voice number. That said, RingMeMaybe makes it awfully convenient to get multiple numbers on the run, and for the nominal charge of a buck apiece.