It's certainly well-known that you can setup file sharing very easily in the Sharing panel in System Preferences. But you don't have to open System Preferences to share files. In fact, you can share files with anyone, even without creating an account for that person beforehand.
In the Finder, select whatever folder or file you'd like to share and choose Get Info from the File menu (or right- or control-click and select Get Info). Expand the Sharing & Permissions section if it's not expanded already at the bottom of the window. A lock icon should appear in the lower-right corner of the Get Info window; click it to authenticate. After you enter an administrator username and password, click the plus (+) symbol to display a user-selection dialog. Besides seeing users already created on your Mac, you'll also see the Address Book along with any groups you've created. In addition, a New Person button lets you simply extemporaneously add a new account right there, without the need to schlep back to Accounts pane in System Preferences.
Any account you establish in this manner defaults to read-only privileges on the folder or file you've shared. The account that gets created is a Sharing-Only account (new in Leopard).
One final tip: if you delete the account in System Preferences without removing the account in Sharing & Permissions for your file or folder, then you'll be left with a share called (unknown). Re-add the user you deleted, remove the share for that person (select the user's name and click the minus (-) symbol), then delete the account in System Preferences.
Mac OS X Hints
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