Flywheel is a an app that does the same thing as Uber Taxi (your city has to have Uber Taxi, most smaller cities like Jax just have Uber X and Uber Black), and it has the support of the taxi unions. Unfortunately, thus far it's only in SF, LA, and Seattle.
Those I know, including myself, who take cabs frequently use Flywheel instead of Uber. The cab drivers often thank me in person because Flywheel skims less off the top for the driver and they receive a higher tip (which I can set...anywhere from 0% to 25%+).
That said, I was just in Paris and it's a super easy city to hail. The cabs are often brand new Mercedes, so for me I actually preferred to hail a cab there than to use an app or Uber X.