For small business and lower volume email:
AWeber: An excellent blocking and tackling email solution for the small business. A favorite of the affiliate marketing industry. I particularly like the auto email feature that will turn a site's RSS feed into an automatically populated newsletter. Close monitoring of the customer base ensures high clearance rates. Perfect for the website owner looking for an easy to manage CRM solution.
For high volume email distribution:
Campaign Monitor: Elegant and simple service for the high volume emailer that requires design tools, email distribution and tracking support.