Sasa

Keeps an eye on what you're after: it watches product pages, ticket pages and registration pages, and tells you when something restocks, drops in price, goes on sale or opens for preorder, with the link and what changed. Pages are checked every few hours, which suits restocks and launches, not split-second drops. You can add conditions, like below a certain price or only in your size; anything that should happen next (a reminder, adding it to your cart or filling in a form on your connected computer) becomes a routine triggered by the change, and paying or submitting is always left to you.

• The white IKEA KALLAX shelf is sold out at the Emeryville store and online. Keep watching and tell me when it's back, with the link.
• Tell me when Amazon Japan opens preorders for that limited-edition Nintendo controller. The page is in Japanese, so check that it's genuine and whether it ships to the US.
• The moment NYC Marathon registration opens, tell me and fill in the form on my computer. Show me before you submit.

Add it, then pick one to start its chat with.