One of the most delightful holiday gift products you can get the gamer in your life is the Critical Hit LED d20 from ThinkGeek. This is a large plastic d20 that blinks red red you roll a natural 20.
The LED d20 is totally awesome; however, in practice it’s not terribly practical. The die is awkwardly large and all the electronics inside make it pretty clearly unbalanced. You need a good amount of space to roll it (I’ve found the best way is to spin it in the air, since it won’t roll much on the table).
For day to day gaming the Critical Hit d20 it’s terribly practical; however, for special occasions, for GMs running one-off con games, and for showing off it’s awesomeness to your friends the LED d20 is a pretty sure-fire hit for any gamer.