Notify users when item is back in stock
This is not currently possible out of the box with Commerce at time of writing (Sept 2016).

You could write a custom plugin to do it, which would basically keep a table of these people-to-contact, and run a task (or use a controller you trigger by cronjob) -> to send out the emails at such a time the product is re-stocked.  

It wouldn't be spectacularly difficult to implement.  Here's a rough sketch:

A form on the site to accept the email addresses -> that form posts to a controller in your plugin - which stores a record of the product, customer email, and date emailed (date emailed is initially null).  

You then have a task that periodically runs and checks these records - for each record, if there is no date emailed, check the stock of the product - if greater than zero, send an email and log when it was sent.