codecademy's Learn Python: The first lesosn under each unit is free
(shouldn't even need an account) and allows you to program right
in your browser. The first lesson (~20 problems each) under Units 1,
2, 3, 4, and 5 are a great introduction to Python.
codecombat has created a web-based RPG-style game that requires
learning Python to advance through the game. The main "campaign"
is free and shouldn't even need an account. Link:
"Learn Python the Hard Way" is an online book that goes
through Python via exercises of increasing difficulty. The online
HTML-based book is free and there's no need for an account. Link: