After a few stops and starts (get it?!), Hailo is here, and it wants you to hail yellow cabs from your phone, for any of your rainy-day, late-night, I-forgot-to-DVR-Castle-I'd-better-get-home-quick needs.
It's super-simple -- it geolocates you (though you can move your pin to adjust the pick-up spot), and shows you surrounding cabs complete with a handy ticker displaying how long 'til pickup (so far it seems to take 2-4mins). You just click "pick me up here", and holy crap, a cab shows up. It'll even wait for you for up to 2.5mins, because nobody should ever ride in a cab without buying a donut first.
For now, all payment is done in the cab, although e-payment is coming soon, and here's maybe the best part: they don't hit you with ANY premium if you pay in cash, and even a charge will cost "less than what you'd tip a doorman at a hotel". This is basically why they invented the iPhone.