Steam API JSON function
Upon coming face to face with the mighty cross-origin request block, and needing to get a JSON file from an external source, I decided to use the Yahoo Query Language in order to achieve this.


Using YQL just to get a JSON and parse it sounds like a dirty workaround.
I don't know the right answer about that part.
You might be better off asking on stackoverflow.com instead,
to overcome your original problem without resorting to such workaround.

About the code posted, these are only minor nitpicks:


Where does key come from? Perhaps it should be a method parameter
User lowercase for variable names (site (or url) instead of Site)
You look up the 'Steam64ID' element twice. It would be better to do it once and store in a variable to reuse, for example:

var steam64ID = document.getElementById("Steam64ID").value += "" + app.id64;
getFriendList(steam64ID);

The function is called getID64 which sounds like a getter, so it should return something, but it actually doesn't.