I was going to build a website for a friend's restaurant, I was wondering how can I implement an online ordering system. Anyone know of any pre-built user friendly systems out there?
I'm not sure if this meets your needs, but you might want to try OpenTable. They just went public so (despite their name), I'm not sure if they're open source.
We have a hosted system that is free for the restaurants to set-up themselves, or you can go with our enterprise system that starts at 49.00 per license at ezWaiter.com
Best is to try Virtue Mart plugin from joomla - I am working on it, it works really well. This well serve your purpose of ordering food online, plus it also has options to integrate it with paypal and other payment gateways.