A few weeks back Google released "Add-ons" for Google forms. These are free scripts that you can add directly into a form to perform certain tasks. They can do things like eliminate options once they're selected from a multiple choice question (think how awesome that would be for conference sign ups!) and lock a form once a certain number of responses have been submitted (think if you're having sign ups for a club).
These can all be selected from a little drop down menu up the top of the form creation page:
This will then bring up a mini-app store where you can manage your add-ons. There's not many of them right now, but they're (mind the pun) "adding" more all the time:
I'm doing a couple of blog posts/videos about some of these that will be on the website soon.