Slightly off topic though already sort of mentioned. What about a free closed source version of Stack Overflow for download?

I'm thinking along the lines of things similar to Microsoft's 'Express' editions of Visual Studio where the product works but has certain limitations built in.

You could for example create a free to download but closed source 'Stack Overflow Express'. It could be limited by for example, only working with SQL Server express (total database size 2GB Max), no support for running ads, limited number of page views a day, limted number of total users, limited styling support, no plugins, etc etc.

This would make a great free product for interanet or extranet use within small companies, organisations or groups and not impact on the business of getting the big users to buy the real thing.