Python: fecha y hora actual

#!/usr/bin/python
import datetime
now = datetime.datetime.now()
print now

Result:
2013-08-03 01:32:27.711721


Para formatear la salida de la fecha:

import time
datetime.datetime.now().strftime('%d/%m/%Y')

Ref.:
http://www.pythonforbeginners.com/basics/python-datetime-time-examples/
http://www.tutorialspoint.com/python/time_strftime.htm