if you type: "su <i>username</i>"
where username is the name of the user you want to switch to, and then you press enter, and then you type in their password, you will be able to control terminal based on their priveleges. <b>this is not full access to the comp</b>, only to terminal.
it's also probably not what you wanted, but it's a step in the right direction...
bo_dilly.tripod.com (a site for dummies!)
<i>this is not full access to the comp, only to terminal.</i>
Not actually the case either. At the prompt, type
open -a <application>
where <application> is the name of whatever app you want to run. Whoever you were in that terminal, that's who the app runs as. So, su root, and then open -a TextEdit (I don't know if capitalization matters). Now you can edit config files owned by root, in a graphical editor.
I believe that the app has to be in the /Applications directory for open to find them