Not sure how YQL (Yahoo Query Language) has escaped my attention for so long, as it seems so useful.

YQL lets you query any of the Yahoo's APIs using a SQL-like syntax. For me this came in handy when I wanted to create a simple stock-tracking mobile app.

In JavaScript I could get the stock quote for Google's stock as JSON, like so:

function getStock(symbol){
  var YQL="select * from yahoo.finance.quotes where symbol ='"+symbol+"'";
  var URL="http://query.yahooapis.com/v1/public/yql?q=" + escape(YQL) + "&format=json";

  //Do an AJAX call to the above URL to get the JSON back.
}

To see what the JSON look like see it on the console.

When it comes to using it in mobile apps, if you use Titanium then you can use the YQL wrapper and write code like this:

Titanium.Yahoo.yql(
 "select * from yahoo.finance.quotes where symbol = 'GOOG'", 
 function(e) {  
  var data = e.data;
  //Do what you like with the data JSON object
 }
);

All very useful.