Whilst this might a very well-known Photoshop tip, I’m going to share it anyway. When you drag a guide out from the ruler, holding down SHIFT forces the guide to snap to the ruler units – if you’re zoomed well in then they snap to pixels – which is perfect when you’re mocking up grid-based website layouts.

Hope that helped someone, it certainly helped me when I discovered it recently!

EDIT: You need to have View > Snap activated for this to work.