Right click on the display in Calculator.app and select 'Large Type.' An overlay appears with the number in large type so you can see it easily. Simple, but cool.
This works in the Lion version of Calculator.app. I don't know if it was there in earlier versions. It works just like the 'Show Large Type' feature for phone numbers in Address Book.
Here's some bonus Calculator.app trivia: Mac OS X 10.7's Calculator has 16 help pages.
[crarko adds: Also, when you right-click in Calculator the same pop-up menu gives you the option to change over to RPN if you prefer. I tried it in Snow Leopard and it's not there. It's always a nice touch when the older applications get some improvement.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110730233116739