Below are links to websites with helpful SEO information that have proven to be valuable over the years.

My favorites
SearchEngineLand.com
King of Search Danny Sullivan, brings you up to date with the latest in the search field. Listen to Danny's podcast The Daily SearchCast, for the latest and greatest on what is going on in search. Danny will inform, rant, & sing about everything in the search marketing field. Entertaining, informative and fun!

Webmasterradio.fm
Full of live and recorded podcasts on everything from SEO, the Web, Affiliate Marketing, Advertising and more.

Other good sites
SearchEngineWatch.com
Great site for SEO 101 information. Danny Sullivan was one of the founders of this site and wrote many of the important articles here. Great for beginners.

Wikipedia.org
Good general description that explains SEO in everyday common language.

SearchEngineRoundtable.com
Great site filled with great articles on SEO. Great forums.

DigitalPoint.com
Info, services, tools

BruceClay.com
Bruce Clay was the firm to put out the search engine relationship chart out for free. The firm has a code of ethics, and teaches SEO as well as provides services.

SEOMoz.org
Overview of many of the processes, techniques and strategies used by professional search engine optimization specialists.

SEMPO
SEMPO's mission is to increase awareness and promote the value of search engine marketing worldwide.

SearchEngineJournal
SearchEngineOptimizationJournal.com is an SEO blog

DavidNaylor
Funny, cheeky, with a passion for SEO...

ClickZ
SEM Firms: Search Engine Friend or Foe?
Shari Thurow

Search Marketing Now
Search Marketing Webcasts - free webcasts about Search

SEO Conferences
SMX- started by Danny Sullivan
SearchEngineStrategies.com - originally started by Danny Sulivan
PubCon - Check exhibitors and speakers...top notch