Mactracker has been a TUAW favorite for a while. For those of you not familiar with it, it's an application that lets you search a database of every Apple product ever made, including notebooks, iPods and even printers. Selecting an item shows you everything you could possibly want to know about it, whether it's the original memory it shipped with, the maximum OS it can take, or even the model's original cost in any number of currencies.
The version 6.0 release includes the latest Apple hardware, a new user interface and in-window search. As you can imagine, if you're just curious about your Mac, Mactracker can answer most questions you have about its specs, but if you're an IT specialist, Mactracker is an invaluable tool. Mactracker 6.0 is a free download from the developer's website or on the Mac App Store.