I can't emphasize this site enough. The Python Challenge has a special place in my heart. I was a bored C# .NET developer. In my downtime I would poke at a challenge. I learned Python through this site; picking up the standard library as the challenges progressed. From my experience with this site I was able to land a Python webdev position.

pythonchallenge.com changed the course of my career and I'm grateful for it.

This is a good one. I was going to post it. It's good that I checked before I duplicate the same content.