AngelList

LeadFerret is the worlds first 100% online business database with complete information, including email addresses. Users can search millions of records for any type of contact or business using a robust set of search tools, and see complete information for all results, including email addresses, titles, SIC codes, phone numbers, company website, and much more.

Think of it as yellow pages on steroids, or the most expensive pay-for-data site you can think of, but completely FREE!