A basic ROT13 implementation in Python

rot13.py

#!/usr/bin/env python2.6

import sys

from os.path import basename
from string import lowercase, uppercase, maketrans

if len(sys.argv) != 2:
    print 'Usage: %s text' % basename(sys.argv[0])
    sys.exit(1)

ROT = 13

rotated = [letters[ROT:] + letters[:ROT] for letters in (lowercase, uppercase)]

table = maketrans(lowercase + uppercase, ''.join(rotated))

print sys.argv[1].translate(table)