CTRL + ALT + SHIFT + S - Save for web... probably my most used - because of this I've also assigned it to F3.

Have also set up some others commonly used by me, so not that useful if you don't set them up yourself :P but I use F keys for Edit>Transform>Resize and for a recent project that needed me to reformat 1,500 images I added F key shortcuts for Actions that added/removed set amounts of Contrast & Brightness.