This would indeed be a great feature.
I would also include number of visits, search engine market share, all the reports under Visitors > Settings.

We could also have the Piwik administrators specify a Category for their website which would allow to give breakdown of these metrics by website category.

The reports could be available on the piwik.org website in a "Internet Report" page that would lists the number of opted in websites being tracked, total traffic, and the various available reports. I think we should build this as a plugin installed on piwik.org/demo that would receive all the opted in metrics and build the aggregated reports.

If anybody is keen to build this, please shout :) This sounds like a very interesting data intensive project.

Data we could also share

• Visits per month
• Page views per month
• New visitors per month
• Average time spent on site
• Average time spent per page
• Average page views per visit
• Percentage of direct requests
• Percentage of visits from search engines
• Percentage of visits from sites

see also #4589 Use Piwik to track how Piwik is used! Share data publicly with community