Finding people in the net through InfoSpace is an easy searching. It can search a person's phone no., email addresses, physical addresses simply or by reverse lookups.

Yahoo's Four11 is an older directory online also does a pretty good jobs finding people's website on the Net.

Who Where? searches personal information with email addresses and phone numbers.

Bigfoot also does the same giving phone, email and websites addresses. Bigfoot services rather stronger than the others.

Now, almost every search engine having its own email lookup function but as they are not maintaining their own email directory, they get the search done from Four11, WhoWhere etc.

In reality, it is not possible to keep information of all the people in the world for any database. Therefore, some directories are probably very large database manipulating tool, some other may keep more up-to-date information handling tool etc. and if you cannot find your search result in one, you hence need to go for looking up into another and so on.