I wanted to create a saved search (aka smart folder) of all papers -- PDF and Postscript -- I've downloaded in the last week. Unfortunately, the saved search functionality provided by the Finder didn't seem powerful enough to express this search: I could search for PDF files, or I could search for Postscript files, but not both. This hint describes how to access the more powerful expressive power of Spotlight.
Spotlight queries can be expressed using a simple query language, described here. The canned search criteria in smart folders use only a subset of the language. You can experiment with the language by using the command-line tool mdfind; you can also see example queries by invoking "Get Info" in the Finder on the saved search file. See the webpage above for more details.
To use the query language in a saved search, add a new criterion to the search and click "Other..." for the search attribute (the first field of the criterion, usually something like "Kind" or "Last Opened"). Then select "Raw Query" from the list of search attributes. Enter your query in the text field.
For example, to create the new papers query, I created a saved search with the criteria:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050501190159752