Search the Current Folder
As far back as Tiger, when you searched for something with the search bar in a Finder window, it would default to searching your entire computer. With Snow Leopard, Apple finally added the option to change the default to searching in the folder you're currently in.
To change it, go to Finder -> Preferences and then to the "Advanced" tab. Then, on the drop-down menu for "When performing a search:" select "Search the Current Folder":