Ahrefs
Back-link index, keyword research and rank tracking
Ahrefs is a paid SEO platform built around its own web crawler and back-link index. Site Explorer shows a domain's referring domains, anchor text, top pages and estimated organic traffic, which is the main reason people pay for it: this is competitor data you cannot get from free tools.
The suite also includes Keywords Explorer with volume and difficulty estimates across several search engines, Site Audit for technical crawling, Rank Tracker, and Content Explorer for finding pages on a topic. Ahrefs offers free Webmaster Tools for sites you verify, which gives limited back-link and audit data at no cost. As with every third-party tool, treat traffic and volume figures as estimates for comparison rather than as measurements.