Frank's Cyber Cafe

PayPal

Single download buttons can be made in PayPal. After payment is made it asks where you want to go! Just be sure and encrypt the button to your downloads page.
Use a script, there are plenty of sites that will encrypt URL's emails etc...

DownLoad Guard

Not just a download page protector, DLGuard is a full sales and customer management system and can help you protect, streamline, and grow your online business!

Secure single product sales
Built-in shopping cart
Create Membership sites
Expiring download links
Limit download attempts

Offer promotional coupons
Calculates sales conversion rates automatically
Template based for easy customisation
Automatic mailing list sign-up
Integrated with the major payment systems

PHP-IPNMonitor

Sell digital downloads with PayPal and Google ....
PHP-IPNMonitor is a PHP / mySQL application for selling digital downloads (such as software programs, mp3 files, graphics, icons, photos, ring tones, ebooks, videos) from your web site. It can also sell physical (tangible) products as well.

The application provides the checking of the completed transaction from PayPal, secure downloading of digital goods, the sending of confirmation emails, and a shopping cart to display the items.

In the case of PayPal, a download url is sent to the customer by email. In the case of Google, the download url is displayed to the user on completion of purchase.

The customer does not have to have a PayPal account to purchase an item but can use any Credit Card supported by PayPal. PayPal then becomes just a payment gateway for the credit card.

See Downloads and E-Commerce for more info