Informed Delivery is a USPS service (external link) that e-mails you when letter-sized mail or packages are going to be arriving soon. I think it may be only first-class letters but any package should trigger an e-mail. Quite an interesting free service that allows anyone receiving important mail or those traveling to keep track of their post from anywhere they have e-mail. You can also manage package mail through Informed Delivery.
The only downside is that it is a somewhat lengthy process to first set up. Figure on fifteen minutes or more to go through all the registering. Once done, however, you’ll see something like this when a letter is on the way, either by e-mail or by logging onto your Informed Delivery dashboard. By this picture, I see I owe someone a Christmas card. You don’t get a picture of a package, though, as far as I understand, just a notice.
It’s not working in all areas yet but you can check on availability before heading into the registration process.