Over the years we have tried and/or subscribed to dozens of SEO tools. Here are a few that we like and use on a weekly basis:
Majestic SEO provides valuable backlink data for research and analysis. Majestic SEO uses Citation and Trust flow as a metric for rating backlink profiles.
We wrote a review of Majestic SEO in our blog if you’d like to learn more.
Open Site Explorer by MOZ is another valuable backlink research tool.
Moz has its own set of unique metrics to look at: Page Authority and Domain Authority, among other great tools in their PRO toolset.