Is there a Drupal module to add a Creative Commons license to entities?
Yes, in addition the the Creative Commons licensing support that is integrated in the Media Gallery module, there exists a module for that does exactly that: the Creative Commons licenses module. From the project page:


  The Creative Commons module allows users to select and assign a Creative Commons license to a node and any attached content, or to the entire site. It also provides integration between CC and Drupal technology.


It was originally written for D6 and is still under development for D7 (up to alpha2 at the time of writing), but the core functionality is in place.

Somebody may ask: 


  "Why would you need a module for that? Why can't you just add an Entity Reference field to link to whatever license the user chooses? What tasks should this module accomplish that a simple dropdown (or dropdown with link) cannot?"


The answer is that there is a lot more to Creative Commons licensing than just linking content to the CC license.

I think that reading the road map for the D7 port will shed some light on what is desirable, and why you need a module to accomplish it. To summarize: When licensing content under Creative Commons it is highly desireable that RDFa and ccREL metadata is properly presented, and you need more than a link for that. A CC search option should also be is available.

Looking at the issue queue for the Creative Commons module, it looks like development may have stalled when it comes to porting the more advanced features of the D6 version to D7, but I think it still is worth checking out.

There is also an experimental module named Attributions. It is not only for Creative Commons. It lets module and themes authors who use third party materials (e.g. fonts, photos and icons) that requires attribution register an attribution string in the project's .info-file. All the registered attribution strings from various projects can then be shown together on an attribution page or block.  Disclosure: I am one of the co-maintaners of Attributions.