The term "cruft" is hacker slang for digital detritusobsolete, extraneous, or otherwise useless files that have accumulated on your disk over time and now do nothing but take up space. By removing cruft, you can recover valuable disk space, increase the speed of backups, file searches, upgrades, and disk diagnostics, and reduce the chance of software conflicts. If you have a brand-new Mac, this step may not apply to you, but even a few months of use can generate a surprising amount of cruft.
Determining which files you need and which can go may be a nontrivial undertaking. Some files ("My 2006 tax deductions.xls" or "Take Control of Maintaining Your Mac.pdf") are obviously important, and some (caches, old downloads, and so on) are obviously disposable. In between you may find thousands of files that you can't identify and that may or may not have some value.
My advice is to work slowly and deliberately, and avoid deleting anything whose purpose you're uncertain about. In particularwith only a few exceptions I'll mention shortlyyou should be circumspect about deleting things in /Library, and almost never delete anything in /System. And remember: this is something you do to reduce clutter, not a matter of life or death. So don't be too ruthless when it comes to deleting files.
Back up first: Because you're about to delete files, I strongly recommend that you make a full backup first, in case you accidentally delete something important. See Create a Backup System and Back Up Everything.
Here are my suggestions for files you might consider deleting:

• Your ~/Documents folder is a likely place for unneeded files. Skim the contents of this folder and its subfolders, looking for documents and application support files you no longer need, and drag such items to the Trash.

Tip

Numerous programs make automatic backups of their files. This is a good thing, but over time you might accumulate dozens or hundreds of old, large backup files that do you no good. BBEdit and MYOB AccountEdge are examples of programs that tend to generate large numbers of backup files.
In addition, if you save iChat transcripts (in ~/Documents/iChats), you might also wish to delete old ones. And Eudora users may want to look through ~/Documents/Eudora Folder/Attachments Folder for unneeded attachments.

• Look in /Applications (and /Applications/Utilities) for any software you've installed over the past year but never use. (Expired demo software, anyone?) Resist the temptation to delete Apple software that came with Mac OS X, though; you may need it later.

• In the folders /Library, /Library/Application Support, ~/Library, and ~/Library/Application Support, look for folder names matching applications you no longer use, and delete them.

• Your /Library and ~/Library folders may hold other folders that store components of third-party utilities. Look inside the folders called Application Enhancers, Bundles, Contextual Menu Items, InputManagers, and PreferencePanes for any system enhancements you no longer use, and drag them to the Trash.

• Third-party Dashboard widgets live in ~/Library/Widgets. Any widgets you don't use can go.

• Software that requires some component to be running in the background all the time may install folders in /Library/StartupItems. In most cases, you should leave this folder alone, but if you see anything there from software you're sure you don't use, delete it.

Warning! The /Library/StartupItems folder often contains background software you need but weren't aware you needed. For example, SOHO Notes uses an item in this folder called OpenBase; Retrospect uses a folder named RetroRun; and Now Up-to-Date & Contact uses a folder named NUDC. In short, if you're uncertain about anything in this folder, don't touch it.

• Kernel extensions (files with names ending in .kext) add low-level functionality to Mac OS X. Examples include hardware drivers (for devices such as mice, trackballs, and audio interfaces), encryption tools, and screen-capture software. These files are stored in either /Library/Extensions or /System/Library/Extensions. Any such software you no longer need can be deleted, but be very careful, especially in /System/Library/Extensions: most of these files are essential to Mac OS X, and that includes some that may have a third-party company in their name. If you see any obsolete items in one of these folders, the safest way to remove them is to run the installer that put them there in the first place and choose Uninstall (or follow uninstallation instructions provided by the developer). Do not delete them manually.

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Uninstaller Utilities. If you prefer not to muck around in your Library folders looking for individual files to delete, consider picking up a utility that can do all the hard work for you. Here are some examples:

• Spring Cleaning: This utility from Allume (www.allume.com; $50) can find and remove all sorts of cruft, including empty folders, orphaned aliases, Internet caches, and of course ordinary applications and their supporting files. Among the many tricks up its sleeve is the ability to restore items it has deleted if you later realize that you need them.
• AppZapper: A much simpler tool, AppZapper (www.appzapper.com; $13) does just one thing (and does it well): it removes all the pieces associated with a given application, including preference and cache files, items in your Application Support folders, and even installer receipt files.

I should also mention two utilities that don't make any attempt to uninstall software but simply help you identify and delete large files on your drive that you may not need:

• OmniDiskSweeper: (www.omnigroup.com; $15)
• WhatSize: (www.id-design.com/software/; free)

Tip
If you hover your mouse pointer over an item in the Login Items list for a few seconds, a yellow tool tip appears with that item's complete path. This information may not tell you exactly what the item does, but it at least tells you where it is, which may provide important clues.

You may find a few unfamiliar items in the Login Items list that are nevertheless legitimate and useful. In particular, do not remove these items, if present:

• iCalAlarmScheduler: Enables iCal to display alarms even when the application is not running.

• iTunesHelper: Watches for an iPod being connected or disconnected, to help iTunes communicate with it.

• Microsoft AU Daemon: Schedules automatic updates for Microsoft Office applications

• System Events: Enables AppleScript (or applications based on AppleScript) to send commands to parts of the operating system.