OS X - TodoApp with shortcut that can popup on any space/desktop? I'm fairly new to the Mac world. I really love the multiple desktop feature. 

Unfortunately, every todo application I've tried takes me to the regular desktop when I want to use them, or on their own desktop. I would like to have an app that could pop-up on top of the screen no matter where I am – like Spotlight when I press Control + Space.

I'm not sure it is possible though, since it feels like each app in full screen mode takes control of the whole "shortcut space". But once again, I don't know anything about OS X.

Is it possible, and if so, is there a todo app I could use that would do that ?
Try Wunderlist for Mac. It has a feature called "Mac Quick Add" where you press ctrl+alt+w, and you can quickly add a todo with spotlight-like interface, and it works on any active space as long as the app is open.