A handful of the best sites in general on marketing, IMHO.
iMedia Connection
Bills itself as connecting the marketing community. Excellent set of resources on “new” media.
Marketing Experiments Journal
Discover what really works. An online testing laboratory for all things interactive. For example, what differences do small variations in copy make on the effectiveness of landing pages? Membership is free although the site is always looking for companies to sponsor specific research topics.
Marketing NPV
Tools and techniques available for download to support quantitative marketing.
Marketing Profs
Marketing know-how from professors and professionals. Some portions of their site are available for free, others to premium-members only for a fee.
Marketing Sherpa
Practical know-how and case studies. Excellent site. Detailed research reports are available for a fee.