I learned from LifeHacker that...

USPS Mobile Tracks Packages from Your Cellphone

USPS' (United State Postal Service) mobile friendly website is found at...

http://m.usps.com/

The timing of gaining this knowledge was timely for me personally. I learned that a package mailed to me with tracking information is listed as delivered although I did not receive it. Thanks USPS! Sheesh...

And, oh yeah, as LifeHacker notes, whoever designed the mobile page for USPS didn't fit it quite correctly for mobile devices.