Last updated 13 February 2013.
See also the Policy on 3rd Party Libraries
While technically the GPL permits inclusion of code with GPL-"compatible" licenses in a GPL package as explained here, the Drupal policy is not to mix licenses. Drupal founder Dries explains this policy as follows:
Drupal.org's package management system automatically adds the GPL license to all packages. If we allow other licenses in Git, it is going to get messy, and sooner or later we're going to run into licensing issues. Already, we get quite a few questions about Drupal's license. If we are going to add more licenses to the mix, it is going to be harder to audit, or provide answers to such questions. So, not allowing other licenses in Git is a deliberate choice.
We've also decided against mirroring other projects in our Git repositories--unless there are good reasons to do so. So when people need a non-GPL library, it is best to instruct them to download that library from that project's website.
So, in short:
- Only GPL'd code/images/files
- But no GPL code/images/files that are available from 3rd party sites as long as the proper version to use with your module is easily accessible. Easily accessible means that the release that works can be downloaded in a compressed file of some sort. If your users have to use svn/cvs/git to get the files then that is not considered easily accessible.