Well familiar with Constant Contact and generally pleased, we took some time to explore their competitors.
Below are a few.
Mainly to be rebellious, I am going to try Mail Chimp for a little while. The principal reason is that I am testing BatchBlue as a CRM with a social media crawler tie-in. BatchBlue has a pretty seamless interface and sync function. In a nutshell I have a single customer record to maintain regardless of whether I am on my iphone, in gmail, in BatchBlue, or Mail Chimp. I think this review also summed up pretty good Mail Chimps benefits over Constant Contact. http://www.cloutsmiths.com/2011/mail-chimp-vs-constant-contact/
Here is another nice comparative review
Giancarlo Newsome is the founder of G-Force Accelerated Marketing.