Python: Functional, One-liner
factorial = lambda n: reduce(lambda x,y: x*y, range(1, n+1), 1)
NOTE:
- It supports big integers. Example:
print factorial(100)
93326215443944152681699238856266700490715968264381621468592963895217599993229915\
608941463976156518286253697920827223758251185210916864000000000000000000000000
- It does not work for n < 0.
