Auto responders are the key to doing business on line and the most appropriate one to use is largely determined by what you want to do with it.
My personal favorite is
Aweber which provides great delivery rates and has strict user guidelines to ensure that you don't breach the can spam regulations. It is easy to use
I have lately been introduced to a very good product called
TRAFFICWAVE which is comparable to Aweber but not quite as user friendly, it has an advantage over Aweber for Internet Marketers as it's business compensation plan really makes it a wortwhile product to promote.
Finally, if you want to load a heap of leads into an auto responder and commence a legitimate email marketing campaign you may consider
AutoResponse pLus . Autoresponse plus has a one time fee and is loaded directly on to your domain, it's worth considering but is not quite as functional as te other two.