Is ginger a good substitute for galangal in Thai green curry?
Scottish,

Use equal amounts of ginger to substitute for galangal.  And yes, it is the best substitute available from your average non-Asian grocery store.

If you can get your hands on dried, powdered galangal, however, you can do better.  Add about half the amount of ginger, and around midway through cooking add a teaspoon of powdered galangal for every tablespoon of fresh in the recipe.  The combination of fresh ginger and powdered galangal will be very close to fresh galangal in flavor.

Also, if you make a field trip to an Asian market, peeled galangal cut into chunks freezes quite well.