AlienVault Open Threat Exchange (OTX)
The OTX DirectConnect API allows you to easily synchronize the Threat Intelligence available in OTX to the tools you use to monitor your environment.
Web Of Trust (WOT)
Developers can build web services on top of the WOT API. Search engines, information security companies, social media, and other industries can benefit from reputation data to enhance user experience.
The VirusTotal API lets you upload and scan files or URLs, access finished scan reports and make automatic comments without the need of using the website interface.