I know that removing an equipped shield and stowing it (usually on the back) is a standard action.
I also know that dropping an held or wielded item is a free action.
What action type is removing an equipped shield and let it drop to the ground?
In general, it is a standard action to remove your shield from your arm. From the Rules Compendium pg 267:
If you choose to remove and stow it on your backpack, that is described specifically on pg 245 (Equip or Stow a Shield ) as also being a standard action.
It appears the hard part is unstrapping it from your arm. Putting it someplace once it's off is the easy part.
My understanding is that your shield is actually strapped to your arm. Therefore to drop it you first have to unstrap it.
Unstrapping it and stowing it is a standard. I would probably require the same for dropping it. I feel like the reason for making equipping and unequipping a shield a standard is to make it difficult and I think that downgrading the action isn't in the spirit of the mechanics here.
However, the logical downgrade would be to a move action, this would provide you with some value because to get your shield back would then require a minor action to pick it up and standard to equip it.