Inventory Hunter

The general idea is if you can get notified as soon as a product becomes in stock, you might have a chance to purchase it before scalpers clear out inventory. This script continually refreshes a set of URLs, looking for the "add to cart" phrase. Once detected, an automated email is sent, giving you an opportunity to react.

it runs on your own hardware, so no processing time is spent servicing other users
you get to choose which products you want to track
you are in control of the refresh frequency