A unified package tracking website. Put in some tracking numbers, get a centralized list, get status updates for all your packages from a single source. There's one site I've found that sort of does this (packagetrackr.com), but quite frankly it's awful.
I also have some ideas for premium services: forward emails and automatically extract/add tracking numbers to your account, SMS notifications, a few others. I just have to pull the trigger and actually make it.
As someone who gets a metric butt-ton of packages, my prototype has been super useful. I actually have it automatically parsing my mailbox and extracting links already, which is really convenient. Where possible I also extract any product names and display them next to the tracking number.