For those of you that are interested in developing your own investing apps, I just tumble upon this resource :
The IEX stock market has just released a totally free web API that allows you to query real time data regarding stocks on US markets :
More documentation here :
It seems that so far only data for US markets is available
And to give even more credits to these guys, Brad Katsuyama is the guy who fought successfully High Frequency Trading by creating IEX. The full story is available in Michael Lewis’ book, Flash Boys. (And if you don’t know Michael Lewis, go and read The Big Short, Boomerang or Liar’s Poker, you will learn a lot on the finance world).