You have webspace that you dont use (say from some free web directory). Many folks need server space for files, etc. You sign up for a service like seti@home where your spare webspace is placed in a "cloud" where others can use it for some set time. In essence a huge virtual webserver space is created
which is an amalgam of all the heterogeneous locations.
So when you visit a site created from this "cloudspace", images could be on many different servers, files on others, etc. etc. The software would be something that keeps track of where all your assets for your site are, and even have "backups" in alternative locations.
The incentive for the donator of the space might be ad revenue based on space, or on the ability themselves to use the service..
This is similar to an older Idea of creating a progam that allows you to create a virtual webspace that is a collection of all the free webspaces you sign up for, plus any free ftp space, etc. etc. but all that is hidden behind a virtual big drivespace.