All of the major operating systems support multiple users (even the iPad is getting it), which allows each user of a machine to have their own settings, wallpaper, bookmarks, etc. Usually, you have one log in and that is it, but I’ve been experimenting with having two logins, and switching between them depending on the task. I now have a personal login on my laptop and desktop, and also a work login. That way, I can focus on work stuff from that login without getting distracted or interspersing personal sites and emails with my work.
Under OS X, visit the System Preferences… and select Users & Groups.
After authenticating by clicking on the lock in the bottom right, you can click the + to add a new user.
I’ve been making my new users administrators on the machines, but that’s a personal preference. By not making them administrators you’re increasing the security of the machine.
Under Windows, new users can be added through the control panel, and Linux with its control panel. With quick user switching, you can jump between the accounts without having to log each one out. When I was running Linux, I ran each workspace on a different virtual terminal, so I could switch with a keypress (wish I could do that with OS X…).
If you have others that use your machines, such as your own children, creating their own log in allows you to separate their preferences and files from yours, while also securing the machine by not making them administrators. When users are not administrators, they cannot install software or mess with the configuration of the computer.