Is there any way to expose Lightning .app publicly ? I enabled Lightning components on my dev org and was thinking to do it via force.com sites but I couldn't find a way. Perhaps there is a way to put the Lightning app inside a visualforce page somehow and then in this way I can share the VF page on a Force.com Site?
At this time Lightning Apps are only for SF1 Mobile. Edited to add that they're single page apps, so embedding them would not be possible anyway.
@everyone as part of spring 17 Lightning components can be added to unauthenticated sites using lightning out. below is the link
ltng:allowGuestAccessinterface to your Lightning Out dependency app to make it available to users without requiring them to authenticate with Salesforce."
Technical implementation details are mentioned in the above link.
Just to add detail to the answer as the question did specifically mention a force.com site.
The org needs to be communities enabled. Based on that, I'm inferring that you can use lighting.out only against a community and not a 'solo' force.com site. It seems force.com sits are being encouraged to sunset IMHO.