Below are some great valuable sites that no marketer can do without. The sites are invaluable but they also have newsletters or Discussion lists that are first rate.

Webbers Communications
The eMarketing Digest

JimWorld
Includes the former VitualPromote site and The Gazette
You should also bookmark the GetHigh Forums

Adastro

Adlandpro Discussion List While you are there, list a free classified, reaches 9000+ sites.