I don't know how useful some of them are, but they can be interesting nonetheless.
Search for People on the Internets, on the InternetsRapleaf
http://www.rapleaf.com/Searches social networking and other sites (such as MySpace, Facebook, MyYearbook, RockYou, Yahoo360, Friendster, PalTalk, Flickr, Plaxo, Amazon Wish List and some others I'm forgetting) for accounts using the email address you search for.
MySpace Profiles
http://myspaceprofiles.org/profiles/FRIENDID.html(Obviously replace FRIENDID with their friend ID)
View MySpace profiles without detection (Doesn't trigger profile view count or trackers) and/or without all the horrible glittery graphics which makes G*d kill kittens.
Search for People IRLLOLZabaSearch
http://www.zabasearch.comFree people search. Information updates are lagged ~1 year, though.
Domains & IPSIpNeighbors
http://www.myipneighbors.com/Whois Source
http://www.whois.sc/WHOIS, DNS, DMOZ and Wikipedia llistings, Alexa, Quantcast and Compete ranks, traceroute
Hey RBCP! Do you think there'd be enough interest and discussion to warrant a separate board for PI topics?