This Critical Hit D20 Light-Up Necklace listed at $19.99, but you can get it for $4.99 during this clearance sale at ThinkGeek. Even with a $5.95 shipping fee, which is free on orders over $75, that's the best price we can find anywhere. The necklace has a power switch, so you can light it up in the evenings. The batteries are included (and replaceable). Looking for other nerdy gifts? There's a clearance sale going on with many more options to choose from. Sales tax is charged in most states.