forms

1. An autoresponder

or

2. An email list script

The difference between the two is very gray, as many new programs/services fall half way between the two, or have functions in both areas

Traditionally an auto responder is a special email address, which a customer sends a message to, to ask for information, and which replies automatically with one or a series of messages.   Nearly every autoresponder system also allows you to put a form on your web site, so to sign-up, instead of sending a message to the special address, a customer enters his email into a form on your web site to sign up

Traditionally an email list script, is like a list of people, where a customer can subscribe in some way (similar to autoresponders) to your list, (and later unsubscribe if he doesn't like the  messages you send him.

Once you have either of them, what you can do is one of two things

EITHER
in the first message you send them the download info

OR
after they enter the address in the form, they are taken to a "thank you" page on your web site to get the download.

The first is probably better in my opinion, as it makes sure people enter genuine email addresses

As I say there is a VERY gray line between the 2, as many services have functions in both areas.

Both types of services can run in 2 possible ways

EITHER
- You can run it from your own web site.  Usually these are CGI scripts to install.    Opt-in lightning (primarily focused around email lists, but I think has some autoresponder functions too) is the best known example of this type of product.

OR
- Somebody else can run it from their server, and rent you out your list or autoresponder service.   Aweber's autoresponder (which also has some list functions too) is probably the best known example of this.

Now there are tons to choose from.  THere is a long list of autoresponders here:
http://www.suniltanna.com/autoresponder.html