Your opponents can't cast spells with even converted mana costs. (Zero is even.)
Your opponents can't block with creatures with even converted mana costs.

8/25/2015 Yes, your opponent can’t even. We know.

1/24/2020 After an escaped spell resolves, it returns to its owner’s graveyard if it’s not a permanent spell. If it is a permanent spell, it enters the battlefield and will return to its owner’s graveyard if it dies later. Perhaps it will escape again—good underworld security is so hard to come by these days.