If this is switched on and you have been dragging folders of music into iTunes, it will have been duplicated; iTunes will be playing the version it placed into the music folder, not the original. In this case, locate the originals by running a Find and delete them.
If this option is off, iTunes will be playing the originals, wherever they are, so be careful what you delete. Deleting tracks from playlists in iTunes doesn't remove them from the library.
To permanently remove them you need to go to the main Music list at the top-left and delete them from there. This also applies to videos which you can delete from the Movies section, and both techniques must also be used on iPods to permanently delete material, unless you have syncing switched on.
Podcasts can also become terrible space hogs, so manually delete these from the Podcasts section, and also use the Podcast Settings menu to make iTunes only keep recent episodes. If you want to back up music, podcasts or videos, either drag them to Finder and burn to disc, or burn via the File > Library > Back up To Disc command in iTunes.
How to find large files using WhatSize
1. Download and install WhatSize from the disc. The license allows unlimited measuring and the free trial allows up to 20GB of measurement. Physical volumes are shown at the top-left and major folders displayed beneath them. Select a drive and the program will begin to measure it. Folders shown in red are waiting to be calculated.
2. Folders are listed in order of size and shown in Column view so it's easy to spot the space hoggers. Here, for example, the GarageBand loops are taking up over a gigabyte of space. If you want you can select a folder and use the delete command in WhatSize, though you should be careful not to throw away anything vital to the system.
3. There are several views available including a pie chart option to show you how space is used. Laptop users note: the large sleepimage file in private > var > vm is normal and does not need to be deleted. This is a file equivalent to the amount of RAM that your laptop uses to preserve data in the event of a battery drain.
Freeing up space
Once you have identified the culprits that are eating up space on your Mac's hard drives, it's time to do something about it.
Some data you can delete, but before deleting you may want to back it up, either for safety reasons or because it's data that you still need but you don't need immediate access to.
Old work and email files, old pictures and finished projects are all examples of material that needs to be archived. If you are running OS X 10.5 you will be able to take advantage of Time Machine, which performs incremental backups and will let you find items even if they've since been deleted from your main drive.
Time Machine, however, is more about security of backups than archiving data, so there are some other, better techniques to use to achieve this.
Burn data to disc
All Macs come with a disc burner and most from the last few years will be able to burn DVDs as well. Dumping data off a hard drive onto discs is a good way to create a "hard" copy for archive purposes, though it's advisable to get better quality discs and burn them at lower speeds to minimise the chance of any errors.
CDs typically hold 750MB, some up to 800MB, but for any media files like video or music, DVD is the better option simply because a standard disc holds 4.4GB. When Blu-ray drives are introduced to Macs, which surely must happen soon, you'll be able to get 25GB on a disc or 50GB on a dual-layer disc.