I could spend all day listing the reasons I love Gmail, but instead will list one I just discovered a couple of days ago. In my inbox, I received a shipping notification for a package shipped via UPS. I opened the email and on the right hand side saw:

Would you like to...
Track UPS package
{Tracking number here}

Track UPS package is a link that takes you to the UPS websites Package Tracking page for the package where you can see its updated status. That is so Alan Cooper. For more information on this cool aspect of Gmail, see this Help Center link.