I'm occasionally surprised by the vast number of SE sites that exist by now, and I feel that I'm losing the ability to judge correctly where to put my questions. This issue is probably shared with a lot of people that are used to only a few sites and do not spend a lot of time on sister sites.
I believe it is more and more required to direct users to the correct site in the network. I'd suggest 2 ways to achieve that:
When posting a question, try to guess the correct site from the caption, text and tags. For instance, a question tagged "google" might rather belong to WebApps than to SO, so display a div message asking the user if he would like to post it over there instead.
Provide a generic question interface that uses the above mechanism to judge the question and display the most likely sites it could belong to. The user can then pick a reasonable choice and does not have to memorize all existing sites. For instance, if I were to create a question "Is beer and pizza a good combo?" (sorry for the stupid example) it might propose Cooking and Homebrewing.
This obviously needs to be a lot more sophisticated in reality. Anyway, I strongly believe this is very helpful to all users of the SE network. While each site should be able to live on its own, the proposed additions may continue making SE itself a huge and still easy to use Q&A directory.