set white_list to {"Finder"}
try
tell application "Finder"
set process_list to the name of every process whose visible is true
end tell
repeat with i from 1 to (number of items in process_list)
set this_process to item i of the process_list
if this_process is not in white_list then
tell application this_process
quit
end tell
end if
end repeat
on error
tell the current application to display dialog "An error has occurred!" & return & "This script will now quit" buttons {"Quit"} default button 1 with icon 0
end try