D2
A d2 isn't a die - it's a coin.
You flip the fucking thing. Heads count as 1, Tails count as 2.
You can, alternately, roll any other die - counting odds as 1 and evens as 2.
In Various Works
d2s are used with disturbing frequency in both CCGs and RPGs. They are used in Magic: The Gathering, and even Dungeons and Dragons because some faggot developer thinks it's fucking funny to have certain items and weapons do damage using d2s even to this fucking day.
Also Currency
The d2 are the only dice you can put in a vending machine and spend for candy.