How can I make an application open at startup, without automatically opening a window? Many of the applications that I use, such as Spotify, don't require a window to be open to function. I want these applications to be available at startup, but if I chose the corresponding options, the window also opens, and I end up with a clutter of windows on my desktop which I end up manually closing.

Is there a hack or a simply option to just open the Application without creating a new window?
Try System Preferences > Users and Groups > Login Items and press the little + sign to add your application.

To hide an application on startup, IIRC you can unselect the "Show on Startup" checkbox.