Building a list by offering software is the best viral type of marketing you can do.  People love software and put large value into software.   Software has much more value than a downloadable ebook or report.

Add in your autoresponder code and set an activation word that the user will have to enter to use your software.  Once you have your activation code, create your first autoresponder email that will contain the activation word once they sign up for you list.

Copy and paste the autoresponder html code for your list into the software.  Now you have a automated way of building your lists.  Especially if you offer resell rights or master resell rights to your software and let other marketers build your lists for free.