This Instructable is another cool way to substitute for a standard 6-sided die, and reminded me of the extremely geeky method we use. A great teaching tool for kids, too.
Step 1: Acquire Coins
Coins are readily available.
Just find three visually distinct coins: I've got a dime, nickel, and penny here, but you could mark up three of the same coins with a Sharpie to distinguish them if necessary.
Step 2: Understand Binary
Designate one coin for each of the following:
20 = 1 (assigned to the penny)
21 = 2 (assigned to the nickel)
22 = 4 (assigned to the dime)
Heads = yes, tails = no.
Flip (or shake and dump) your coins and add the values to figure out the value of your roll.
Step 3: Scoring Examples
H H H = 4 + 2 + 1 = 7 (discarded- reroll)
H H T = 4 + 2 + 0 = 6
H T H = 4 + 0 + 1 = 5
H T T = 4 + 0 + 0 = 4
T H H = 0 + 2 + 1 = 3
T H T = 0 + 2 + 0 = 2
T T H = 0 + 0 + 1 = 1
T T T = 0 + 0 + 0 = 0 (discarded- reroll)
(For a d8 the TTT=0 is considered an 8, and none are discarded.)
Check out the examples below, then go roll dice like an MIT nerd.