Python is the official development language of the Raspberry Pi. The complete language is already present on the official 'Wheezy' Debian Linux-based operating system image (see the Pi downloads page).
Python has a clear and easy-to-grasp syntax. Its many powerful features enable us to create useful programs with only a few lines of code. In addition to a rich collection of built-in functionality there are hundreds of specialised modules.
Modules to help you write code for tools, utilities, games, websites, smartphones, robotics, and much more. There's also a friendly and highly active community, who have created thousands of useful code samples, tips, Wiki help pages and 'how-to' videos.
As Python is free to use and distribute it's found in software companies, research laboratories and academic institutions across the world. Google developers, astronomers, robotics engineers, space scientists, nuclear physicists and bioinformatics researchers all have used Python.
A post from my Learn Python on the Raspberry Pi tutorial.