I’m all for offering options with your onsite search but I came across a great example of how you can mess it up. The dutch site Knowington offers books for sale, so logically they offer a way to search trough their inventory.

A simple search box nothing special… When you try to use it a few usability problems come up which make using the search annoying. Especially the extra options that appear when the search box gets focus (with the mouse cursor).

Unexpected extra options
It’s unexpected because I didn’t ask for more options but I did get them. Only by focusing on the search box (I clicked the search box) the drop down menu appears.
It’s not working properly
Now this is a hard thing to say because the search function works fine and the selective search also works fine. However, they made the whole background of the header a homepage link. This with the search box not being that large results in a lot of clicks back to the homepage, deleting anything you might have typed in the search box and removing your selection search preference. Of course this is also a problem when you first try to click on the search box.
Even though your focus (the cursors focus that is) stays in the search box I can imagine a lot of people clicking the search box again. I did the exact same thing, since the type cursor disappears from the search box when the options pop up.
The solution is simple as it gets: either show the options right away or show a more options link. Don’t confuse visitors.
All basic stuff, maybe they should buy one of their own books on usability.