Blend modes are the way a layer affects other layers under it. For post-production work, I use just a few of them on a regular basis, depending on what effect I want to achieve or what kind of output I'm working with.
The four blend modes I use for enhancing 3D images are the following:
Normal: This is the default blend mode. It doesn't affect other layers under it.
Multiply: In this mode, the darker areas will darken the layers under it; pure white areas don't effect the areas beneath it. Everything else dims the image beneath it to some degree.
Screen: The opposite to the Multiply mode. It brighten the lightest areas, while leaving the dark areas untouched.
Overlay: A mix between Multiply and Screen. It brightens the lights and darkens the shadows. It's a very strong effect, raising the general contrast.