Can I start an application as another user without actually switching users?
From the command line you have to find the Unix executable and run it using sudo like so:

$ sudo -u Bob /Applications/Firefox.app/Contents/MacOS/Firefox 


… or the same with su: (however this also needs you to set up a password for root - which I do not recommend)

$ su Bob
$ /Applications/Firefox.app/Contents/MacOS/Firefox


(…or the above in one line:)

$ su Bob -c /Applications/Firefox.app/Contents/MacOS/Firefox




In the past, the easiest way for PPC Macs was to use Pseudo, but it has been long discontinued. This is to drop the application icon on Pseudo's icon or window.