Apple pioneered the idea of a Desktop containing files and folders with the earliest versions of the Mac OS, and the basic idea has not only remained but also been adopted by practically every other computing system.
On a computer, it's hard to tell at a glance if a machine is chock-full of stuff. The Desktop might be neat and tidy, but just a few folders down the system could be overflowing with files.
Get the info
The Get Info window is your best friend when it comes to looking at the sizes of items on your Mac. You can select any item and press [Apple]+[I] to view its properties.
You can also view an item's size if you Quick Look a folder or drive in OS X 10.5 by pressing [Spacebar]. If you open any Finder window and choose List view, you can click the Size column to order the items by size, though you may have to activate View > View Options > Calculate All Sizes to force OS X to display folder sizes initially.
It may take a moment to update itself, but this will give you an overview of the biggest folders on the system, which are likely to be your User, Applications and Library folders. Within your User folder, probable culprits for clutter include your Documents folder plus all media folders – Music, Pictures and Movies. The more you work with any particular media, the more likely that folder is to be full.
OS X and the apps that run on it sometimes decide where to store things on the hard drive, because, in most cases, you wouldn't want to be prompted every couple of minutes and asked what you thought. These things build up over time and take up valuable space.
There are several usual suspects when it comes to gradual build-up, but they are easy to identify. If you use Apple Mail, you can soon accrue a lot of mails and, with attachments, these can start to take up more space than you realise. Go to your Home folder > Library > Mail and perform a Get Info on the Mail folder to see its size.
You may also see a Mail Downloads folder, containing those attachments you have chosen to save using Mail's Save button. Note that this isn't the best place to delete mail, just to view its size. To delete messages, open Mail and drag the relevant messages to the Trash folder, then empty it using Mailbox > Erase Deleted Messages or by right-clicking on the Trash folder.
To sort the messages and see all those with attachments – likely to be the largest in size – right-click in Mail's Subject bar and choose to view attachments, then click the new Attachments column to sort the list. Remember that if you're unsure about deleting mail, you can drag the messages into a folder on the Desktop first and burn it to a disc.
Microsoft Entourage keeps its mails in a big, monolithic database file, so the same procedure is necessary – drag mails to the Trash in Entourage and empty it.
Keep an eye on iLife
Each iLife app has its own Trash bin. So even if you empty the main Trash, deleted material from iLife programs may remain. To delete this material, locate the Trash in iPhoto, iMovie, iDVD and the others and manually empty it. Sometimes the Trash isn't obvious – in iDVD, choose Advanced > Delete Encoded Assets.
iTunes is a space hog; music, videos and podcasts linger even if they're never played. Find out whether you have iTunes set to copy music when you add it. Go to Preferences > Advanced and look for Copy files to iTunes Music folder when adding to library.