[Updated to include instructions on how to configure web browsers to only use Tor for some websites but not others.]
Tor is an EFF supported open source software project that makes it difficult for anyone to figure out who a Tor user is talking to on the Internet. For example, someone using Tor can pretty effectively hide which websites they visit, where they download content from, who they are sending e-mail to, etc. As I explain below, Tor is a tool everyone should be interested in, even those who don't think they have anything to hide.
Unfortunately Tor's performance can be quite slow. But using proxy configuration files (pac files) it is possible to configure browsers to use Privoxy/Tor for some websites but not others. This is not a perfect solution since, as I explain below, there are some trivial ways to get around this technique but it is better than nothing.