If like me you use an audio interface that has a separate application for setting up the inputs, gain and other stuff then you may find the way the app disappears under Pro Tools infuriating when you are using it. The same can be said for some third party apps such as RX that you may use in conjunction with Pro Tools.
Afloat is a nifty little app that means you can lock it on top of the session until you don't need it any longer, even if you click into Studio One the focussed application will stay afloat. You can even set transparency if you wish.
I use it with my Apogee Maestro app for when I need to set things up.
It's free and works like a charm.