Tape measures. I must have at least 8 around the house and a couple in the car. Why? One 5m tape measure is enough for any use in the house or on the games table - maybe a spare in case your opponents have failed to bring one). But 8!? OK, a couple are small compact ones that slip but I have 4 which are essentially exactly the same - 5m tape, black outer.
Perhaps one day I'll be involved in an 8 way battle where everyone else has forgotten their tape measure and I can save the day.
Otherwise....