I'm a heavy user of iTunes's smart playlists, which are dynamic listings of tracks based on criteria you define; as your iTunes library changes, smart playlists automatically update their contents to reflect those changes. For example, instead of creating a standard playlist called Recent Jazz, and then manually adding new jazz tracks to it and removing older tracks from it, you can create a smart playlist with the criteria Genre Is Jazz and Date Added in the last 3 months. Whenever you add new jazz tracks to iTunes, that playlist is automatically updated to include them; tracks you added prior to three months ago are pruned automatically.
But if you're a fan of smart playlists, you know that managing them can be a hassle. For example, if you want to create a new playlist that's similar to one you've already configured, there's no way to just duplicate the existing smart playlist and tweak the copy. Similarly, if you tend to start most of your smart playlists with the same few criteria (for me, Genre is not Holiday Music and Kind is not Video are common ones), you can't create a playlist template and use it as the basis for new smart playlists. Finally, there's no easy way to export just a smart-playlist configuration--say, if you want to use the same playlist on multiple Macs, or if you want to give your favorite, complex smart playlist to a friend. (iTunes's Export Playlist command exports the list of a smart playlist's contents, as well as its criteria; when you try to import it on another computer, you get a warning that the tracks are missing.)
The typical workaround for the first two challenges is to create "base" smart playlists--in my example, one that excludes all holiday music, and another that excludes videos--and then use those smart playlists within other playlists. But this clutters the iTunes sidebar with extra items, and it slows iTunes's performance--the more smart playlists you have, the more work iTunes must constantly do to keep them all updated.
For smart-playlist junkies, Smarts, by Doug Adams of Doug's AppleScripts for iTunes fame, is a handy (and free!) utility that works with iTunes to provide a few of the smart-playlist features many people have long wished iTunes itself offered.
Launch Smarts, and its window shows two lists: Current Smart Playlists includes all the smart playlists that currently exist in iTunes; Saved Smart Playlists includes a few sample "starter" templates, such as Four Stars Or Better and No Holiday Genres, that you can use as the basis for new smart playlists. Rather than mirroring iTunes's folder hierarchy and sort order, the list of current playlists is a flat list, sorted alphabetically. While I initially didn't like this, I found that it helped me identify long-forgotten smart playlists, hidden in folders, that were good candidates for removal.
Select a current playlist, and you can save it as a new template (with the same or a different name), reveal it in iTunes, or delete it (which deletes it from iTunes). Select a template playlist and click Load, and you're prompted to give the new playlist a name; you also have the option of immediately editing the new smart playlist. Click OK, and iTunes opens with the new smart playlist selected and, if you enabled the option, with the playlist-editing dialog open for tweaking. (Note that when you use Smarts to add a playlist to or remove a playlist from iTunes, Smarts temporarily freezes. This isn't a bug; it's Smarts waiting for iTunes to update the iTunes database so Smarts can then load the updated version.)
I mentioned above that smart playlists can bog down iTunes. If you've got smart playlists you don't use regularly, but that you don't want to permanently delete, one solution is to edit each of those playlist's settings to disable Live Updating, which reduces the amount of ongoing updating iTunes must do. But the easier solution--and one that has the added benefit of cleaning up iTunes's sidebar--is to save those unused smart playlists to Smarts and then delete them from iTunes. If you want to use one again later, it's easy to use Smarts to load it back into iTunes. For example, Macworld contributor Kirk McElhearn often uses smart playlists to listen to audiobooks he's ripped from CDs. When he's done with an audiobook, he can save it to Smarts before he deletes it, just in case he ever wants to listen to it again.
The smart-templates list also lets you add descriptive notes to playlist templates--useful for helping you remember exactly what a particular saved playlist does--as well as delete templates and export templates to XML files that can be imported into other copies of iTunes as smart playlists. Because a smart playlist contains only the playlist's criteria, and not the tracks that match that criteria, its exported XML file is only about 4kb in size, making it easy to copy to another Mac or email to a friend. You or your friend can then use iTunes's playlist-import command (File: Library: Import Playlist) to add the smart playlist to any version of iTunes.
One minor hassle here is that to export an existing smart playlist, you must first save it to the templates list and then export it from there; you can then delete it from the templates list. Sharing playlists would be a bit easier if you could export directly from Current Smart Playlists. But perhaps the biggest drawback of using Smarts is that when you load a playlist template, or import an exported smart playlist into iTunes, the playlist's view options--which columns are displayed and in what order--are set to iTunes's defaults. This is because playlist view options are stored within iTunes's preferences, rather than within each playlist.
Nevertheless, if you make use of smart playlists in iTunes, Smarts is a no-brainer download from the Mac App Store.