In my particular case, I have the original account I created when I first isntalled OS X, and I recently created an acount for programming and web serving. These accounts belong to different UNIX "groups," so when I think up a solution to a programming problem while surfing the web in my original account, I can't simply open a Terminal and cd to the other account. However, from the same terminal, I can do the following:
% su other_account_nameI will be prompted for the password to the other account, and thus logged in as the web server/programming user. What's more, since I've stuck with the original prompt that Mac OS X has set up, it lets me know that I'm the web server/programming user. Typing exit at any point brings me back to my original account.

