Notim: Back In Stock+Notify Me

This saves me a LOT of spreadsheet time. It notifies me on a schedule I determine what products I'm out of, so I know what to make without having to do a weekly physical inventory of hundreds of items. Works great, no problems, and depending on the package you get, you can customize it further to tell you all kinds of stock related info.

Honestly, worth it just to not have to deal with the endless spreadsheeting.