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.)
Displays files in iTunes Media folder not in iTunes track library
Enter text for the selected tracks' Category tag
Edit a selection of iTunes tracks' tags in single window
Paste non-ID3 Tag track and file related info into Category, Comments, or Description tag
Perform search-and-replace on text in your choice of tags
Delete characters from the beginning or ending of selected tracks' name, artist, album, comments, composer, or show tags