• PyFlakes - haven't used it. Supposedly rather lite.
• PyChecker - haven't used it. Supposedly it actually runs your code

PyLint#

You can get it at http://www.logilab.org/857

Pep8#

You can get it at http://pypi.python.org/pypi/pep8

Sample usage

$ pep8 optparse.py
optparse.py:69:11: E401 multiple imports on one line
optparse.py:77:1: E302 expected 2 blank lines, found 1
optparse.py:88:5: E301 expected 1 blank line, found 0
optparse.py:222:34: W602 deprecated form of raising exception
optparse.py:347:31: E211 whitespace before '('
optparse.py:357:17: E201 whitespace after '{'
optparse.py:472:29: E221 multiple spaces before operator
optparse.py:544:21: W601 .has_key() is deprecated, use 'in'