TAILS -- The Amnesiac Incognito Live System -- is a highly secure operating system intended to be booted from an external USB stick without leaving behind any trace of your activity on either your computer or the drive. It comes with a full suite multimedia creation, communications, and utility software, all configured to be as secure as possible out of the box.
It was Edward Snowden's tradecraft tool of choice for harvesting and exfiltrating NSA documents. Yesterday, it went 1.0. If you need to turn a computer whose operating system you don't trust into one that you can use with confidence, download the free disk image. (Note: TAILS won't help you defend against hardware keyloggers, hidden CCTVs inside the computer, or some deep malware hidden in the BIOS). It's free as in speech and free as in beer, and anyone can (and should) audit it.
Effectively, this is the ParanoidLinux I fictionalized in my novel Little Brother.
Tails is a live system that aims to preserve your privacy and anonymity. It helps you to use the Internet anonymously and circumvent censorship almost anywhere you go and on any computer but leaving no trace unless you ask it to explicitly.
It is a complete operating system designed to be used from a DVD, USB stick, or SD card independently of the computer's original operating system. It is Free Software and based on Debian GNU/Linux.
Tails comes with several built-in applications pre-configured with security in mind: web browser, instant messaging client, email client, office suite, image and sound editor, etc.