This tip has "been around' for a while, but every so often I mention it and people say, "huh?"
If you have a layer active (other than the Background) and a non-painting tool (such as the Move tool) selected, change the opacity of that layer by pressing the first number of the percentage you want. For example, press 4 for 40%, 7 for 70% etc, You do not have to click on the Opacity field first. Press 0 to get back to 100%.
This tip also works with painting-type tools: press the number and the opacity (or strength or exposure or whatever % that tool uses) will change.