This will be a running list to be updated frequently. Please let me know if I'm missing any. Since they keep expanding, the locations column may not be very accurate.
All these cabs are making me think of Berlin for some reason...
|Locations or Cities
get €10 free uber promocode
|San Francisco, San Diego, Chicago, Washington DC, Toronto, Seattle, Boston, Los Angeles
|Lyft aka Zimride
|25 cities across the US, including San Francisco, Los Angeles, and Washington DC
get $10 promocode
|UK, Israel, Russia
|Taxi Mojo / Swishly
|London, Dublin, Toronto, Chicago
get €10 free mytaxi promocode: "TaigeZ1"
In London we used kabbee which is a really polished app to compare and book private cabs for slightly cheaper than black cabs but they do take some time to be dispatched.
Some more sources: http://venturebeat.com/2012/06/06/gettaxi-funding/ http://techcrunch.com/2008/12/16/taxi-magic-hail-a-cab-from-your-iphone-at-the-push-of-a-button/
Update: Oct 5, 2015 I updated this list with MyTaxi and Uber's funding numbers.