[Jolly]

Well, it is a code-golf ;P I'll un-golf a little and explain it though!

Taking input (obviously)

Code:

n=input()

Neat trick using complement and negation, ~-n is equivalent to n-1. Means I can remove the space between while and that. 0 is falsy, and 1-1 is zero, so once this hits one it will stop executing.

Code:

while~-n:

This is using a trick with list indexing. n%2 will either return a 0 or 1, so I make a list of length 2 which has what I want to do based on that. An even number will return 0, so the 0th element should be n/2. An odd number will return 1, so the 1th element should be 3*n+1.

Code:

n=[n/2,3*n+1][n%2]

Pretty obvious, prints whatever n is at.

Code:

print n

Also, @Eclipse, my score is 45 now

This is quite clever to be honest!
However, why doesn't it get an error since Input() returns a string value, but you are operating with integers.