I'm not aware of a ready-made solution for Repository but it should not be too hard to create your own solution using the Reports module.

Which Repository field(s) would you draw tags from?  Name? Summary?  Description?  A custom attribute?  Whatever field you decide to use as a "tag field" would need to be constrained to tags only.  If, for instance, you used "Description" as your source for tags and you enter whole sentences as descriptions then your generated tags will include many, many words you don't want such as "a", "the", "is", "it", etc.  Your best bet is probably to create a custom Repository attribute "tags" and add terms as, say, a comma-delimited list of words.
 
How you handle the resultant tag data to build a cloud will depend on your desired functionality.  Will your tags be "weighted" with more oft-appearing tags listed first (or in larger font, etc)?  Will the tags be clickable?  If clickable, how will you decide what link to use for tags that appear for multiple items?  In short, how do you envision a Repository tag cloud to operate?

Dave-
I was thinking along these same lines.  I figured the REPORTS module could be a decent starting point so I drummed up a sample cloud generator as a proof-of-concept (a very basic generator).  You can see that solution in the following link -- perhaps it will give you a starting point for creating your own solution (or maybe convince you that purchasing a third-party module is worth the expense):
 
   Quickly create a Tag Cloud with the REPORTS module
   http://www.eguanasolutions.com/Default.aspx?tabid=54&EntryID=33

 
Cheers!
-mamlin