Search Engine Watch, started by Danny Sullivan, THE search engine marketing guru! This site covers Search Engine Optimization from soup to nuts, and keeps it all in a very reader-friendly format. There's a public section to the site and a free twice-a-month email newsletter available to all. There's also a members-only subscription portion of the site as well as a complementary members-only twice-a-month email newsletter. The subscription-based products contain much more detail than their free equivalents and the subscription rate is very reasonable--approximately $100 per year for both.

(Note: Sullivan left Search Engine Watch December 2006 - he's started his own company and SEM Web site - see Search Engine Land and Calafia Consulting for more details).