When a prospect shows an interest in your product, if you don't deliver 
what he wants immediately and follow up on him, you lose sales.
No matter what you're selling online, using an automatic responder to do 
personalized follow up, can drastically increase the amount of sales you 
receive. It can also dramatically cut down on manual follow up of your
prospects.

Once a prospect requests your information there are a million and one
things that can divert his attention from it.

For instance: 

His computer crashed, he has to wait several days to get it repaired.
His wife called, her car broke down again. He has to pick her up.
The kids are screaming, the dog is chewing on their favorite toy.
His friend needs a ride to the airport...now!

These are just a few examples of what can happen to cause your 
prospect to forget about your offer. That's why it is so important to get 
your offer in front of a prospect more than once. The ideal number is 5-7 
times.

How an automatic responder (autoresponder) works

These programs automatically return a prewritten message to anyone who
sends a message to the autoresponder's email address. They usually allow
for one or more follow-up messages.

Types of automatic responders

Single follow up - this one will reply once only to any message sent to it. 
These are very useful for letting someone know that you have received 
their mail and will get back to them shortly, or you could inform people 
you are away on vacation.

Multiple follow up – these are often called sequential autoresponders.
They can be programmed to follow up over a period of days, weeks, or
months with any number of sequential messages.

Features:

• Send multiple follow up messages
• Messages can be personalized
• A copy of each requestors message can be sent to your e-mail address.
• Can create an online form to insert in your web page.
• Supports both plain text messages or HTML.
• Broadcast email updates to your entire customer list
• Attachment support - your messages can contain one or more
  attachments. 
• Tracking URLs - setup a tracking URL to track click throughs from your 
  messages
• Totally Ad Free
• Automatically adds an "Unsubscribe" link at the bottom of each message.

Free automatic responder
autorespond-it.com (contains all the above features)
limitlessfollowup.com (free perl script)
sendfree.com
getresponse.com
freeautobot.com

Paid automatic responder
rapidreply.net