MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Neuro - Instant infrastructure for machine learning
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Filestack - Simple file uploader and robust APIs for uploading, transforming, and delivering any file into your app. Filestack is a collection of tools and powerful APIs that make it simple to upload, transform, and deliver content.
SolveDNS - SolveDNS works to check your name servers, a records, and mail servers, SOA records for errors, consistency, efficiency and speed using several DNS tests.
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.