Finder allows you to specify a default search location when using the built-in search tool at the top right of any Finder window. Your choices are “Search This Mac” (the default, which seems to annoy most users), “Search the Current Folder” (which is what I’ve always wanted), and the verbose “Use the Previous Search Scope.” This option is under Finder > Preferences > Advanced, and I believe it arrived in 10.6 Snow Leopard.