I’m not sure how I missed this — it’s been in Photoshop since at least CS2 and from some of my Google results, probably far earlier than that.
When we cut images for the web, we use the Marquee tool a lot, but it can be a real pain to hit your corners exactly on the first try — sometimes the pixels you’re using as a visual guide are off screen if you’ve zoomed in far enough to see what you’re doing! The solution (of course) is Transform Selection which gives you scale, rotate, and skew control over your current selection just like Transform Layer without messing with the underlying pixels. Very handy for expanding and trimming those rectangular selections.
Access it via right-click (command-click on Mac) of the selection or the Selection menu.