Python - 31 28 26 24 23 18

Uses boothby's idea of a lambda function, saving 5 characters (or 3 if I need to assign it to a variable):

lambda x:(x*x)**.5

Old Methods

Uses a generator function to save 1 char, so it is necessary to print the value with some function that uses a generator/iterator, like for i in a(b):print(i)

def a(b):yield(b*b)**.5

Saved 2 3 characters by squaring and unsquaring.

def a(b):yield(b<0)*-2*b+b

Old one:

def a(b):yield b if b>0 else-b

Edit: saved two characters by factoring b in.