Hi,
although there is nothing built-in to quit all applications at the same time, here is something that might help you for now:
1) Bring up a list of all applications, by holding down command and press tab
2) Continue to hold down command and press Q to quit an application (right from that app switcher).
3) As you continue to hold down command, press Q multiple times to quit all apps until only Finder is left.
Hope this helps.
/Rupert
PS: I'm sure you could do something like "Quit all apps" with Apple Script.