www.radaronline.com had an article on online snooping.
Some of the techniques are probably common knowledge, and some have nothing to really do with snooping (hiding your tracks is a different topic). This list includes some tips from them and some of my own on finding personal information online.
This information is useful not only to find info on other people but equally importantly to be aware what personal info about oneself is available online and what effort it takes to find it.
Web searching is normally the first method tried. I get better results with
www.alltheweb.com than
www.google.com . It is also useful to narrow down by a location keyword such as country or state or city or school attended.
If a website that is returned as a match no longer exists, try using the google cache link or using
www.archive.org to see what the page used to look like.
www.zabasearch.com often reveals personal information, especially if the person has ever had a US phonebook entry and has links to pay-sites for further information.
maps.google.com or the free desktop app at
earth.google.com has aerial imagery at a high enough resolution to identify parked cars (although it isn't updated very often).