One of the consistent best of the lot would be Aweber...
If your sales/product page doesn't convert well for whatever reason, having an autoresponder work for you might just help you salvage your business. It gives people a second, third, fourth... chance to become more familiar with you and/or what you have to offer. It can help you build a strong rapport which can lead to a long-term relationship and business.
I strongly discourage the use of free autoresponders to do your follow-up because they usually come decorated with other people's ads, causing your message to look pale, unimportant and unprofessional... So, it's a Big NO NO.
It's your business. Treat it and run it like one and you'll earn likewise.
