Learn Python the Hard Way - An example-driven introduction to Python.
How to Think Like a Computer Scientist - Python Edition - A good intro if you're also interested in learning basic computer science concepts.
Automate the Boring Stuff with Python - A fun book that teaches Python while automating some common (and annoying) tasks.
CodeAcademy: Python - An interactive tutorial for learning the basics of Python. (I recommend trying the books above first, as learning to use a text editor or IDE such as IPython or IDLE is an important part of the learning curve.) A good supplemental resource.