SEOmoz recently began offering a paid subscription model for value added content on their site. Rand and I put together a Podcast update on how this is working for SEOmoz, and their future plans.
It's notable that SEOmoz already has more than 100 subscribers, and a larger audience of people who are paying do download premium content. These numbers may seem small, but I see them as significant validation regarding the model and the quality of the SEOmoz content.
In addition, we talked at length about the SEO tools provided, both the free versions, and the value added features available for paid subscribers. Tools like the Page Strength Tool, for example, provide great insight into a page, including detailed information on del.icio.us tags, Wikipedia mentions, DMOZ, and other factors. Just having these things be available on the fly to you while in the middle of evaluating the site of a potential client, or while looking at a competitor's site, without having to check all these places one by one, is a great value.
Another thing available for premium members are extensive how to guides. For example, a guide and a walk through of 10 different keyword research tools, and where to brainstorm valuable keyword ideas. Rand and SEOmoz remain committed to generating the same great content for free that they always have. The premium model is designed for those that want even more.