The Tor Browser Bundle is a freeware package designed to safeguard your web browsing privacy by using the Tor secure network to shield your details from prying eyes. It will run locally or from a USB stick, and as well as a specially configured version of Firefox web browser, there’s also an IM client (Pidgin) in there too, both of which run over the Tor network.
That’s the good news. The bad news is that it’s still not foolproof, and if you really really need to be invisible, you’ll need to undertake a few more sensible and rigorous precautions before and while going online. If I remember rightly, this freeware may soon be superceded by a completely new fork of Firefox which has Tor baked in.
The Tor Browser Bundle lets you use Tor on Windows, Mac OS X, or Linux without needing to install any software. It can run off a USB flash drive, comes with a pre-configured web browser, and is self-contained. The Tor IM Browser Bundle additionally allows instant messaging and chat over Tor. If you would prefer to use your existing web browser, install Tor permanently, or if you don’t use Windows, see the other ways to download Tor.