I disagree that this is useful enough to make it worth adding to the standard library.  Between math.copysign and simple comparisons, I think all the common cases are well covered.  And it's a simple one-line function:

def signum(x):
    return (x > 0) - (x < 0)

Frank, do you have any convincing use-cases that aren't already covered by math.copysign or simple two-way comparisons (e.g., x >= 0)?