What version of Python should I use if I'm a new to Python?

Python 2.6 (and 2.5, 2.4) are what you will find installed on most machines (Linux) and what you will find most code written in. Therefore I'd start with Python 2.6.

It's really going to depend on what you want to do. Generally speaking Python 3 "isn't ready yet", in the sense that few libraries support Python 3. This will end up greatly limiting what you can accomplish with the language as a beginner.

On the other hand, if you think you'll be spending your time on more "pure programming" tasks as you learn your way around, then Python 3 can be a good starting point, since things like strings are wildly different in Python 3.

So, what do you want out of Python right now? Do you want to make useful things? Python 2.x. Do you want to learn about Python for the sake of learning about Python? Python 3.

Here in a year or two, Python 3 will likely start being the default choice. Until then, it's really up to you. But you can be certain that whatever skills in Python you pick up are going to be useful in 2.x and 3. Some important things changed in Python 3, but Python 3 is still Python.

The latest 2.6. Don't worry about 3.x; there isn't that much to learn.