Wolf Howl
I solved (at least partly) my problem maintaining rankings over time. Old pages that used to get traffic are once again seeing inbound search visits. I did it by tweaking my blog’s category pages, but I think the lessons are also helpful for ecommerce sites.
http://www.wolf-howl.com/

SEO Book
SEO Book.com is a leading SEO blog by Aaron Wall covering the search space. It offers marketing tips, search analysis, and whatever random rants come to mind. ;) The first version of my popular SEO Book came out in December of 2003. I have probably revised it about 50 times since then.
http://www.seobook.com/bloggers

SEO Moz
SEOmoz provides companies around the world with consulting, Internet marketing and search engine optimization services. Although our home offices in Seattle, WA serve as our base of operations, we're frequently found globetrotting across the US, Canada & Europe. Our goal is to provide a high level of service and education to businesses, individuals, non-profits and government organizations so they can best leverage the unique reach of the Internet to increase visibility and achieve success.
http://www.seomoz.org/blog

Daily SEO Blog
Professional Search Engine Optimization services to give your websites the visibility through search engines and the social media channels. Become an SEO Expert, and get the edge over others!
http://www.dailyseoblog.com/

Phoenix Realm
Phoenix SEO Blog contains up to date Search Engine Optimisation and Web Design news, as well as unique, informative SEO articles and tips that are mainly targeted at the beginner to intermediate SEO level.
http://www.phoenixrealm.com/

Seoptimise
As the Internet has become an intrinsic part of modern business life and continues to grow at an exponential rate, companies are starting to recognise the potential of using it as a powerful marketing tool.
http://www.seoptimise.com/

Seoco
The SEO Company was established in August 2006 by David Eaves. We are a small up and coming company with 1 full-time and 2-3 part-time members of staff. Over the last 18 months we have successfully optimised and secured top search engine rankings for over 30 clients.
http://www.seoco.co.uk/blog/

Bigoakinc
Big Oak SEO was founded in 2004 with the idea that every business can benefit from using the Internet to reach out to clients and promote sales. We aren't just.
http://www.bigoakinc.com/blog/52-seo-tips-advice/

97th Floor
97th Floor is an advanced SEO Firm that specializes in Search Engine Optimization, Reputation Management, Social Media Marketing and Blog Optimization. We are privileged to currently be working with companies such as, Omniture, Dentrix, Elance, Stephen Covey, Home Depot, NHance, Namco Bandai, CityPass, and many other industry leaders.
http://www.97thfloor.com/blog/