50 is the number of known Mersenne primes and perfect numbers.
50 is the number of US states.
In googology Edit
In Greek-based number-naming systems, 50 is associated with prefix "pentaconta-", and with prefix "quinquaginti-" in Latin systems.
It is also the prize for correctly answering the first question in the German game show Wer wird Millionär? in euros.
In chess Edit
There is a fifty-move rule preventing endless chess games.