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.)
Create a playlist for each entry of a specified tag
Add or subtract a user-entered number from each selected track's Plays
Change video kind and video related tags of selected video tracks
Re-associate batch of dead tracks with files from selected folder
Parse Song Names and reassign the parts to other tags
Create a PDF booklet containing info from the selected tracks