pythonselect is a tool to set the current Python version; it is included with ActivePython. If you have multiple Python versions installed - say, 2.6, 2.7 and 3.1 - then running the following will set Python 2.7 to be your current Python:
$ sudo pysel 2.7 or C:\> pysel 2.7
On OSX, pythonselect creates symlinks in /usr/local/bin (hence sudo is required) linking to appropriate binaries in your non-system framework Python install at /Library/Frameworks/Python.framework.
On Windows, pythonselect manipulates the system %PATH% environment variable, AppPath and .py/.pyw associations. You will have to launch a new Command Prompt. Windows support is experimental. Note that on Vista and Windows 7, pysel must be run from an Administrator Command Window.
Linux support is on the horizon.