From Chetan Surpur:
Sidestep is an open-source application for Mac OS X that sits quietly in the background, protecting your security and privacy as you browse the web. When you connect to the Internet through an unprotected wireless network, such as at a coffeeshop or an airport, where you don't have to enter a security key, you're putting yourself at risk.
Attackers connected to the same network can easily intercept your unencrypted traffic and log in as you to services such as Facebook, Amazon, and LinkedIn.
When Sidestep detects you connecting to an unprotected wireless network, it automatically encrypts all of your Internet traffic and reroutes it through a secure connection to a server of your choosing, which acts as your Internet proxy. And it does all this in the background so that you don't even notice it. With Sidestep enabled, no one can eavesdrop on your traffic and impersonate you or see what you're seeing as you browse the web.