Often, when you get a shipping receipt email for an online purchase, the shipper will provide the tracking number as a link, so you can easily see its progress. But this is not always the case. Fortunately, Lion's Mail app (and possibly past versions) can help.
Hover the mouse over the tracking number, and a dotted line should appear around it, along with a drop-down arrow on the right. Click that arrow, and, if a menu ensues, select 'Track Shipment.' A Quick Look panel will open with the shipment info (from the website of UPS, FedEx, etc.).
Mail will do this for you whether the tracking number is linked or not, so this hint can be helpful any time you want to check up on the shipment but don't want to switch to a browser to do so.
[crarko adds: I don't have any pending shipments to try this out with, but two people mentioned this feature works in 10.7. Will someone verify if it does or does not in Snow Leopard and post in the comments? Thanks.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110823090532195