1. Majestic SEO – Majestic-SEO provides Competitive Link Intelligence to link building specialists. They believe they are the planet’s most comprehensive backlinks provider, and offer a number of tools to allow access to this valuable information. In addition to their free services, they allow registered users to test their services free of charge by generating reports for sites they control. Registration is free, and their highly competitive subscription plans start at low prices, with 1 month minimum commitment, and offer a number of tools to enable analysis of competing domains to be performed. For Enterprise users, they offer an API service for internal use which enables your developers to integrate their data into your existing suite of reports and applications.