When you are doing creative Photoshop work, you often find yourself flicking through loads of different layer blending modes.
I recommend you learn the basics of multiply, screen, darken and lighten, but the layer blending shortcuts are tricky ones to use in different studio environments as they have changed many times through different versions of Photoshop.
Heres a nice little tip though - you can switch layer blending types in the layers palette by just using your up and down cursor keys once it is active, a feature that DOES NOT WORK on the Mac version of Photoshop upto CS1 and is sorely missed (please note Adobe!).