"New Window" pattern for small utility apps
Some small utility apps (e.g. App Icon Preview, soon Video Trimmer) have a "New Window" primary menu entry which opens a new instance of the app.
I actually have an issue in Video Trimmer where the user found it unclear how to open another video after trimming the first one (the non-obvious options are to close and re-open the program, or to use right click -> Open With in the file manager to open a second instance). Two solutions I came up with are adding tabs (second video opens in a new tab) or opening second video instead of the current video. Both of these have behavioral issues/questions. A "New Window" primary menu entry suggested by @bertob offers a very nice alternative which sidesteps all behavioral issues and also is the easiest to implement. I think it might be worth mentioning it in the HIG.