SEOmoz is a platform for managing, analyzing, and monitoring Search Engine Optimization, Social Media Marketing, Link Building, and various other online marketing areas.
I’ve been an SEOmoz user and customer since long before the unified their tools into a campaign software. I’m not always a “fan”, but there is generally a lot of value in the tools and community.
Their biggest draw is that they crawl the web and maintain their own index, from which they calculate their own metrics which are meant to reflect how a search engine like Google might “see” a page or a website. These metrics can be used to analyze and track the effectiveness of your online marketing efforts. They aren’t perfect, but many people think them an adequate indicator for measuring the success of an SEO campaign.
This same data can be used for researching competitors, potential link opportunities, new tactics, etc. They also have other tools for various common online marketing needs. Keyword monitoring and rank tracking, analyzing keyword difficulty, and they recently acquired FollowerWonk for analyzing Twitter accounts.
The final part of SEOmoz is their user community. They maintain a very active blog, and a user contributed “YOUmoz” blog. For paying customers there is a Q&A section where you can get answers and help for your online marketing.