For background checks, US Search is a useful site. And, of course, using keywords on search engines can facilitate the task of finding interesting tidbits on people. The Internet isn't the only source of privacy-busting information. Up until very recently, the US Post Office freely revealed a person's forwarding address to anyone requesting it. Spurred by tragic stalking cases, this practice has been stopped.
Town halls are great places to cull information from people's lives.
Depending upon state laws, marriage, divorce and death certificates are all public information. Real estate transactions are also available for public review. Even the good ol' public library has a wealth of information that could uncover interesting facts. Protecting your privacy encompasses everything from avoiding annoying telemarketers to defending the most intimate aspects of your life. For areas outside your control, namely hospital records and electronic eavesdropping, unfortunately, there's not much you can do.