Email List Suggestion Please

G'day all,

I need to find some good (free or very cheap) software to allow email subscription to a clients website. They need to capture the emails for later email marketing (when their site and business is in full flow).

Can anyone suggest anything good and easy to use. Need to have the usual "please confirm you are the one requesting the subscription" and/or those little graphics that come up asking you to key in the letters that you can hardly read.