I have been working with MaxBlogPress Magnet WordPress Plugin to attract more subscribers on my blog for about a year now. And though it did not "double subscription rate" I really like the creative ways it gives my visitors to join to my newsletter without becoming to overbearing.
MaxBlogPress Subscribers Magnet works perfectly with most autoresponder systems such as Aweber, iContact, ConstantContact and Getresponse. Getting set up is relatively easy. You start by simply copy and pasting the autoresponder form code from your autoresponder system into MaxBlogPress. It automatically will detect the correct form fields so you do not have to worry about fooling with any code.
One nice feature is the "Form Auto Filler." This will automatically detect the name and email from your visitors and place it into the sign up form. All they have to do is click submit. The simpler you make the subscription process the easier it gets to subscribe the more subscribers you get.
Another feature I like is the "Optin Form Adder." This allows you to place a subscription form in your sidebar and/or under your posts. You can modify the form with your own html to make it look any way you wish.
The next feature is borderline ethical but I think it stays north of the border. It is called "Subscribers from Comment." If somebody leaves a comment on your blog there is a little checkbox that visitors must tick to say they would like to join your newsletter as well as comment. You can also choose just to show a regular form on the comment page as well.
See here to view the rest of the review and to see MaxBlogPress Subscribers Magnet In Action On My Blog.