The first and obvious way to create a global variable is to initialize it straight from the root in app.js. The usual

var myGlobalVar1 = 'this';

when it is placed out of any brackets.
I actually saw a seemingly official sample application with a comment in app.js saying something along the lines “define your global variables here”. It seems like it should work great, and for me it did… until I implemented notifications using gcm.js.
For some reason, when a gcm.js notification is clicked to open the application, if the application wasn’t already running in the background it will crash because all global variables defined in app.js will be defined locally only (see for instance this post on Appcelerator Q&A). I’m not sure what happens, but my best bet is that somehow gcm.js or the functions it uses launch app.js as some kind of local object, so whatever is defined in it in this context is local and not global as expected.

Anyway, so I needed a more reliable way to create a global variable.
A number of options are suggested in the following posts:

• Appcelerator Community Questions & Answers – Global variables in titanium
• Appcelerator Community Questions & Answers – global variable
• Eveolas – Titanium : Passing parameters between windows [How to make things global]
• Using Global Function in Titanium

Some involve adding includes at the beginning of every file, and then I wonder how a modification from one view might modify the value in another one. The method I found the most convincing is to attach your wanted global variable to an already existing, guaranteed global variable, for instance Ti.App. Apparently this isn’t a “recommended” way, but it’s a natural feature of JavaScript so should be fairly future-proof.

So, instead of

var myGlobalVar1 = 'this';

we’ll now use

Ti.App.myGlobalVar1 = 'this';

and voilà. The only problem now is that you variable names are quite longer. Personally I even use

Ti.App.myappGlobal={};
Ti.App.myappGlobal.myGlobalVar1 = 'this';

so that if by any chance Titanium decides to rename Ti.App I won’t have any issue to automatically replace all “Ti.App.myappGlobal” with something else.

Update (2014-12-16): sadly, apparently this trick doesn’t work to create global functions on iOS… FFFS

Update (2014-12-23): I finally found a workaround: if variables and/or functions are created in a separate file, which is then included into app.js using Ti.include(), then they are properly registered as global. The big problem with this solution is that Ti.include was deprecated in Titanium 3.3.0 so I’m afraid it might get removed in a not too distant future :s

Titanium provides a simple way to detect window orientation, Titanium.UI.Window.orientation, but it seems to initialize late. In an application I’m developing, I needed to check the window orientation right at start-up, more specifically I used this:
if(Titanium.UI.Window.orientation==Titanium.UI.PORTRAIT) {stuff}
but this didn’t seem to work.

Running a simple
console.log(Titanium.UI.Window.orientation)
revealed that the variable sadly is undefined at that stage (TBH, I didn’t check if it was defined later since that’s not where I need it).

So I made a custom function, based on window size, which turns out to be defined early enough for me. Here is the function, quite obvious, really:
var isPortrait = function(){
return (Ti.Platform.displayCaps.platformHeight > Ti.Platform.displayCaps.platformWidth);
};

Then just call it with isPortrait() to get a boolean with value true if the window is oriented in portrait and false if it’s oriented in landscape.

Note that I only needed to make the difference between portrait and landscape, I don’t really have an idea to make the difference between properly oriented and upside-down portrait, or between left of right landscape, as the Titanium.UI.Window.orientation property offers (its possible values are: Titanium.UI.LANDSCAPE_LEFT, Titanium.UI.LANDSCAPE_RIGHT, Titanium.UI.PORTRAIT and Titanium.UI.UPSIDE_PORTRAIT)

Testing for a virtual device can be useful for a variety of features not or poorly working in those, for instance push notifications (last time I tried it just seemed to block the execution) or the camera (as mentioned in this post, on a virtual device you could replace the camera with a “fake” one by loading a picture from the disk).

The Ti.Platform.model variable should contain all you need to determine whether or not you’re running on a real device. On the iOS simulator it will return “Simulator”, on an Android emulator it will return something like “google_sdk”. So a way to test for it would be simply:

if (Ti.Platform.model === 'Simulator'
 || Ti.Platform.model.indexOf('sdk') !== -1 ){
    alert('This is a virtual device');
}

(sorry for the poor indentation, WordPress makes it pretty tough :s)

Sources:
https://developer.appcelerator.com/question/63871/way-to-detect-running-in-simulator
http://docs.appcelerator.com/titanium/latest/#!/api/Titanium.Platform-property-model