WARNING: this ProcessWire module is a proof of concept, not something you should use or even install in a production environment. See this thread for context: http://processwire.com/talk/topic/4127-can-assets-folder-be-moved-to-another-domainsubdomain/
This module is a proof of concept for dynamically altering location of ProcessWire assets to point to another domain and/or subdomain specifically configured to serve static content.
http://www.ravelrumba.com/blog/static-cookieless-domain/ explains why this can be a good idea and how to setup such a domain yourself.