"filename:foo type:pdf"
"search phrase type:xls"
"type:xls modified:>=March 2009<=December 2009"
Even better, save your various searches as "search folders" so that you don't have to remember the syntax every time. Then it just becomes what looks like a normal folder, but as you add files that fit the filter in any location that's indexed, they show up in your saved search.
As I said, the
search is powerful, it's just not nicely exposed from the UI.