For administrators and power users, it’s often useful to log in as multiple users on the same web site. However, you can’t normally do this from one web browser – sites keep track of your whole session. Logging in as one user logs you out as the other user.
An obvious, but bulky, solution is to use one web browser for each log in (e.g. Firefox and Chrome). However, I’ve found that Google Chrome’s “incognito” feature works much better.