Here are some of the software used for secure digital delivery when downloading software.
Protect your digital download files from people bypassing your payment system. Create secure download pages, expiring download links and build a customer mailing list automatically. Fully integrated with Paypal, Clickbank, 2Checkout, Stormpay, PayDotCom, Ebay digital delivery and free downloads.
Paypal provides an easy to setup service for receiving payments for your products worldwide. One problem faced by many companies however is how to provide a secure way to deliver digital goods (software, eBooks, art etc) after a successful transaction.
aMember is a flexible membership and subscription management PHP script. It has support for PayPal, BeanStream, 2Checkout, NoChex, VeriSign PayFlow, Authorize.Net, PaySystems, Probilling, Multicards, E-Gold and Clickbank payment systems (complete list can be found here) and allows you to setup paid-membership areas on your site.
payloadz.com is a hosted solution. Basic package is Free includes 40 MB of storage space, upto $100 sales per month.