This seems obvious, but many people forget: If you're using a web browser (e.g., Internet Explorer, Firefox, or Safari) on a publicly accessible computer (e.g., one in a Lab, the Library, or an Internet cafe), someone else is going to be using that same browser not long after you walk away. If you've left that browser running, and especially if you've left yourself logged in to your email account or any other account, then whoever sits down next will have full access to that account -- including any personal information and the ability to send email as you.
So before you walk away, you should always explicitly log out of any account you've logged into. Then (to be on the safe side) explicitly exit out of the web browser (using the "Quit" or "Exit" option, not just closing the window). If you're extra-paranoid, tell the browser to clear cached or temporary files and cookies before you exit out.