Snowball is a web-based application that streamlines the process of offering content in return for a "favor".  In other words, if you're a content provider, you may want to allow the public to download a product, but only if they first carry out some action that promotes the file.  For example, you may have recently written an e-book that you want to offer as a free download but you want readers to first tweet about the book before downloading.

Snowball is built in a modular way, so that the "favors" (called plugins) can be easily written, enabled, and disabled.  Each download option generally corresponds to an interface to a web-based API.  For example, the Twitter plugin might call Twitter's API, authenticate the user, make a post for them, and make their account follow another account.  The plugins are written in such a way that they can be enabled or disabled by adding or removing a single line of code in the plugins.php file.

Snowball is made with the following ideals:
1) Sometimes it's better to offer things for free, as long as it helps you become more famous
2) Any content distribution system can be circumvented, and that's not terribly important if the content is free
3) It should be easy to create, add, and remove plugins so that Snowball can be used with various content types

Snowball is written in PHP and requires the ability to create custom .htaccess files and access to a MySQL server.

A demonstration of Snowball can be found at http://www.colinraffel.com/software/snowball/download, where the content being provided is Snowball itself!

The main repository for Snowball is https://github.com/colinraffel/Snowball

Plenty of information about plugins, installation, configuration, etc. can be found at https://github.com/colinraffel/Snowball/wiki

Snowball is currently in "beta", which means that aside from having bugs, you'll also need to do a tiny bit of PHP editing yourself, and if you make a mistake, there is not a lot of error-checking.  So if things are breaking, triple check the edits and configuration you did.

Installation instructions can be found in the INSTALL file in the snowball root directory.