Clearbit - Clearbit provides Business Intelligence APIs
Hunter - Find all the email addresses related to a domain
Neustar - A trusted, neutral provider of real-time cloud-based information & analysis to the Internet, telecom, financial, media & advertising and retail industries.
Snov.io - More leads. More sales. More revenue.
BigPicture.io - BigPicture provides company APIs for data-driven B2B teams.
Apollo.io - Apollo’s predictive prospecting, sales engagement, and actionable analytics help the teams to reach its full revenue potential.