Doug Adams has launched a great new Macintosh application on the Mac App Store called "Smarts." The app stores the only the criteria which define the Smart Playlists, the smarts of a Smart Playlist, as templates that can be restored later. Further the "Smarts" can be exported for backup or shared with others.
Doug points out that having a lot of Smart Playlists can slow iTunes down, but Smarts allows you to store unused Smart Playlists in Smarts so that can then remove your unused Smart Playlists from iTunes while retaining the ability to reload them later when you need them again.
Another great feature is the ability to store templates that contain frequently used criteria. These templates can act as a starter for new, complex Smart Playlists where you only need to add a minimum of new criteria to make a full featured playlists.
Smarts is now available at the Mac App Store as a free download. Check it out.