I hadn’t noticed this before, but Google has added a site search box underneath the site links in their results.  

Not sure what criteria they’re using to generate this, because it doesn’t work for every site.  The SMH gets it, but Drive doesn’t.  It seems to be based on sites that are listed on Google News and their size.  That’s a total guess just based on the couple of dozen of searches I’ve checked out.

Not sure how I missed this seeing as it was posted on the official blog site.  The official word on what sites it will show up for is:

This feature will now occur when we detect a high probability that a user wants more refined search results within a specific site. Like the rest of our snippets, the sites that display the site search box are chosen algorithmically based on metrics that measure how useful the search box is to users

Not sure what that means in real terms, but as I said, I’ve only seen it show up for sites that exist in Google News.  That may simply be coincidental though, as people are often searching those sites too.

Update: Strike my theory. The search box shows up for gimp.org & ubuntu.org & they’re not in Google News.  I guess it’s just down to the pigeons then.