This script will add or subtract a half-star from the ratings of the selected tracks. Ratings with half-stars appear with "1/2" appended to their ratings (see the screenshot). I highly recommend attaching a keyboard shortcut to this script for faster access.
iTunes 6.0.2 or better (I only discovered this in 6.0.2; it may work in earlier versions).
The scripts have also been posted on the Missing Menu Commands page as separate add and subtract actions.
Also see Assign Half-Star Rating.
(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
Display, create text file listing info of dead tracks
Exports and then re-imports selected tracks' artwork
Export files of tracks in two or more playlists to single folder
Applet plays tracks with a user-set duration of silence between each track
Set choice of various number tags of selected tracks incrementally