This script will apply user-entered text to the "Category" tag of each of the selected tracks. iTunes 5 or better.
(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.)
Selected track plays after current track ends
Play the selected track and stop
Copy selected CD's CD Text to the clipboard
Renumber order of selected tracks
Edit a selection of iTunes tracks' tags in single window
Renumber playlist tracks to play order