These take pre-created playlists and evenly shuffle them together into a new playlist. You can make a "battle of the bands" playlist, alternating between bands. You can shuffle 5 genres (for example, Jazz-Country-Rock-Folk-Metal) so that every 5th tune is a jazz tune. Perhaps you'd like to alternate between new tunes and old tunes, your wife's favorites and your favorites, etc. These scripts make this easy to do.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)
Remove iTunes tracks disassociated from files
Locates pairs of track entries pointing to the same file
Display, create text file listing info of dead tracks
Exports and then re-imports selected tracks' artwork
Renumber order of selected tracks
App displays files in iTunes Media folder which are not in iTunes library