Windows only: Tiny application NumLocker disables the Caps, Num and Scroll Lock keys from an easy-access menu in the system tray.
Using the application is simple enough—just right-click on the tray icon and choose whether you want the key enabled or not, or you can set the key to be always disabled. If you rarely use the keys and don't mind having an extra utility running in the background, it might be worth a look.
If you never want to use the Caps Lock key again, you have plenty of other options: You can use TabsLock to make it launch your browser, turn it into a dedicated minimize button, or just neuter it with a quick autohotkey script. Thanks, Shankar!
NumLocker is a free download for Windows only. Do you ever use the Caps Lock key for anything useful? Tell us in the comments.