Reverse lookups on land lines generally work well (The Official WhitePages - Find People for Free (http://www.whitepages.com), Yellow Pages: Superpages Yellow Pages, Maps, Driving Directions, Weather... (http://www.superpages.com), Find a Person or Business with our free White Pages and Yellow Pages (http://www.411.com), etc.) however they usually don't work for mobile numbers.

Depending on why you want to know it, there may be some legal ways to get it. See the "similar questions" link on this page.