“Can I use your computer to check something for a minute?” asks a colleague / stranger. You could politely refuse, but if you’re feeling generous you could log your mac into the guest user account in about five seconds without affecting your own open applications.
Assuming that you have the Guest account enabled, it’s as easy as clicking on your user name in the menu bar (top right) and clicking once more on the Guest User icon. About five seconds later, the computer loads the guest account which is completely independent of your own user account.
If you don’t have the guest account enabled, it’s a checkbox in Settings -> Users and Groups -> Guest User. If the settings are greyed out, you will have to click on the lock and authenticate as administrator to modify important system settings. There is no harm in having an active guest account. In fact a guest account would be helpful in allowing a lost/stolen mac to connect to the Internet and become trackable with Apple’s iCloud and Find My Mac feature.
Once your guest is finished, log out of the guest account with the Apple menu (top left) and select Log Out Guest. Or the keyboard shortcut Command + Shift + Q. Keep in mind that files created/downloaded by the guest user are deleted when the guest account logs out. Once the guest accounts is logged out, click on your own account, enter your password, and you’ll find everything is where you left it.
This mac tip was written with OS X 10.8 Mountain Lion in mind, but the steps will be similar on other versions.