Alright, I've been using this site for a few years now, and have decided to share it with those of you who may not know about it.
The site is www.packagemapping.com. Very simply you go to the site, and you can track your packages via RSS feeds on a Google map. It supports UPS*, USPS, FedEx, and DHL. In addition if you create an account you can save your tracking numbers to track at any time, and use a multi-map feature to track all your packages on one map.
*There are a few limitations on UPS tracking due to UPS limiting the
number of times per day that the site can pull from UPS.com, however I
find that it is often more accurate and in depth the the UPS site
itself.
Additional features are a Sidebar gadget for Vista users, Firefox and Thunderbird extensions, an iGoogle Gadget, Avg Speed, Travel Time, Travel Distance, etc... There is also a payed option though I've never used it.
Anyways, check it out and see what you think! It's pretty neat to be able to track your packages on a visual map.
Individual package tracking and mapping.

Note: it even gives you the Distance, Travel Time, and Avg. Speed.
Multi-map tracking and mapping

Additionally there is http://isnoop.net/tracking/, which offers a single package tracking feature. While it is allowed to pull from UPS more often, it does not allow you to save the tracking numbers, or to Multi-map.
Enjoy!