How to do this by setting up a custom search on Google Chrome:
1. Go to www.google.com
2. Do a search for
+(site:stackexchange.com AND site:superuser.com AND site:stackoverflow.com) stuff)
(Add in your other favorites to this list.)
3. Search. Copy the URL from the search results. It should look something like this:
4. Now click the "wrench" icon in Chrome and go to Options > General options > Default search > Manage.
5. "Add new..." and enter name + keyword and paste in the above URL. (Ex: name:stack exchange, keyword:stacksearch)
6. Replace the word "stuff" with "%s".
7. Go to search bar and type in (keyword) + TAB. Search.
Voila! Stack exchange custom search!
Please post any better answers here, i imagine there's a plugin somewhere that can do this.