At the moment, when you vote to close a question when it's off-topic, you can only vote to move it to one of these sites:
meta.stackoverflow.com serverfault.com superuser.com webmasters.stackexchange.com programmers.stackexchange.com
But sometimes the question is really a good one, but it would fit some other Stack Exchange site. And new users rarely know the difference.
http://codereview.stackexchange.com http://math.stackexchange.com http://scicomp.stackexchange.com http://cstheory.stackexchange.com http://android.stackexchange.com http://webapps.stackexchange.com
Anyway, I think giving the whole range of Stack Exchange sites in the off-topic option will help reduce the load on Stack Overflow and direct people to right sites.