I think this problem actually requires two solutions:
1. Users who admit they don't know where to put a question:
I think a 'global' location within the Stack Exchange organization such as:
https://stackoverflow.com/questions/ask
would be a good place to a site recommendation system. It would be an undertaking -- as George Edison notes in the comments -- but I'd be willing to help get it off the ground. Statistically, it would be an interesting classifier problem.
2. Users who think they know where a question belongs:
To address this -- the focus of the OP's question -- I suggest the following considerations:
Parsing tags may be the most useful meta-data to decide if a question is on topic; however, suggesting that a question be migrated after adding a tag could become quite distracting. If new users are uncertain of how to tag a question, they may start typing simply to see the new awesome tag suggestion popup. Suggesting a site after each tag would become very distracting and negatively affect the user's experience. Hence, if the question belongs on a different site, the migration recommendation should appear after clicking 'Post Your Question'.
Providing a means to seamlessly migrate a question will lessen the burden placed on the moderators to migrate a question.
A classifier system won't be perfect. So it shouldn't force the user to migrate a question.
Education is important, so if this were implemented, I think the users should see the bulleted list of on-topic from the faq for both the current site and the suggested sites so that they can learn where to place the questions in the future.
Migrated tags won't work, so if the user decides to migrate a question, they should be prompted to retag a question as some sites use one tag naming convention and some sites user another.
So how about a page, much like the CAPTCHA page, which appears after a users clicks 'Post Your Question' and says (Please imagine radio buttons instead of [O]):
After parsing the content of your question, we believe your question might be more appropriate for a different site within the Stack Exchange Network. Please consider migrating it to one of the following:
[O] Super User
[O] Server Fault
However, in our recommendation, we may not be 100% accurate, please feel free to post your question to Stack Overflow (the originally selected site) with the understanding that the community may migrate your question if they feel it would be a better fit and receive a faster/more complete response elsewhere.
[O] Stack Overflow
Please select a site from the list above to post your question.
Then if the user clicks the arrows, they would see expanded descriptions:
After parsing the content of your question, we believe your question might be more appropriate for a different site within the Stack Exchange Network. Please consider migrating it to one of the following:
[O] Super User
is for computer enthusiasts and power users. If you have a question about …
- computer hardware
- computer software
and it is not about …
- videogames or consoles
- websites or web services like Facebook, Twitter, and WordPress
electronic devices, media players, cell phones or smart phones, except insofar as they interface with your computer
- a shopping or buying recommendation
Super User FAQ
[O] Server Fault
is for system administrators and desktop support professionals, people who manage or maintain computers in a professional capacity. If your question is about …
- Servers
- Networks
- Desktop PCs that you maintain in the workplace
and it is not about …
- Networking outside the professional workplace
- Running servers at home for personal use
- General personal computer troubleshooting
Server Fault FAQ
However, in our recommendation, we may not be 100% accurate, please feel free to post your question to Stack Overflow (the originally selected site) with the understanding that the community may migrate your question if they feel it would be a better fit and receive a faster/more complete response elsewhere.
[O] Stack Overflow
is for professional and enthusiast programmers, people who write code because they love it. We feel the best Stack Overflow questions have a bit of source code in them, but if your question generally covers …
- a specific programming problem
- a software algorithm
- software tools commonly used by programmers
- matters that are unique to the programming profession
Stack Overflow FAQ
Please select a site from the list above to post your question.