TabBot

Actually has a wide range of uses beyond just watching for sales items. Note that it only works on https not http. You draw a window around an area on a website, set timespan, and when the area changes.. ding, ding, ding.. you are notified. Works good as a YouTube video notifier. Let's me know when someone leaves a chat room. It's a challenge to select items at bottom of a website because the app menu appears there by default.

Easy to use alert for hard to find/very popular products. Also very helpful when checking for out-of-stock items in online stores that do not offer an alert for every individual product. Love it!