Google Forms sign-up sheet - limits on choice options for multiple choice, check-box, or choose-from-a-list questions.  
With formRanger and a few Spreadsheet functions, when an option reaches the number of responses you set, it no longer appears in the survey...or, automatically start a wait-list for each option  

Update (June 2015): For many situations, the Choice Eliminator Add-on for Forms will do what you need, and is much easier to use than this process. 

You can find information about other Google Apps Add-ons and scripts here.  

Thanks to:
The Form, Spreadsheet, and Presentation have been updated to include details on:
  • how to add alternate text when all options have met their limit (slides 12 -13)
  • how to count Checkbox responses (slide 14)
  • how to create wait-list responses (slide 15)
formRanger is one of several Google Apps Scripts that you will find at New Visions CoudLab. You'll find a formRanger tutorial there.

You can also use formRanger to create a Google Site Sharing Repository with dynamic tagging - choose from a list of tags or add new ones. Many of the same techniques that I use to limit responses are used to expand choices.
   The links:
Parents' Day conference sign-up Form and Responses spreadsheet
  • The Parents' Day form has not been used in real life yet. Test it carefully and adjust it to your needs. See the note on cell B1 of the Choices sheet.
  • This uses formRanger and formMule