Online Python Tutor by Philip Guo.
From the webpage:
Online Python Tutor is a free educational tool created by Philip Guo that helps students overcome a fundamental barrier to learning programming: understanding what happens as the computer executes each line of a program’s source code. Using this tool, a teacher or student can write a Python program in the Web browser and visualize what the computer is doing step-by-step as it executes the program.
As of Dec 2013, over 500,000 people in over 165 countries have used Online Python Tutor to understand and debug their programs, often as a supplement to textbooks, lecture notes, and online programming tutorials. Over 6,000 pieces of Python code are executed and visualized every day.
Users include self-directed learners, students taking online courses from Coursera, edX, and Udacity, and professors in dozens of universities such as MIT, UC Berkeley, and the University of Washington.
If you believe in crowd wisdom, 500,000 users is a vote of confidence in the Online Python Tutor.
I first mentioned the Online Python Tutor in LEARN programming by visualizing code execution
Philip points to similar online tutors for Java, Ruby and Javascript.
Enjoy!