Using separate functions for Project Euler 1
Here's some superficial review. More for testing the site and some comment formatting than anything, but: do create main functions (helps us benchmarkers a lot) and do think that your module can be imported, so docstrings and local variables help. 

# Finds...
###-^ Put this in a docstring

def find_multiples():
    """Finds sum of all multiples of 3 and 5 from 0 to 999 """
###-^ This allows you to "from x import find_multiples,  help(find_multiples)"
    numbers = []
###-^ Avoid globals
    for i in xrange(1000):
###-^ Use xrange if Python 2.x, no need to start at 0 (it's the default)
       if not (i % 3) or not (i % 5):
###-^ Add spaces around operators, simplify
###-^ the boolean/numeric checks
           numbers.append(i)
###-^ Remove trailing ;
    return numbers

###-^ Removed global

def main():
###-^ Allows calling main many times, e.g. for benchmarking
    numbers = find_multiples()
    print sum(numbers)

if __name__ == '__main__':
    main()