The rectangular version of the "Quick Lasso Fill" or "PolyFill" tool!
I couldn't find one so I made one myself.
Snapping is on by default, because that's very useful, but you can turn that off in the settings.
With this you safe some clicks when you want to just block out some stuff, no need to use the rectangle select, then fill, then remove selection. Drag and fill a rectangular space instantly instead.