This week's favorite is a must-have reference for all Apple fans. Mactracker has been around for as long as I can remember and is indispensable. It provides exhaustive information on every Apple product, from the latest Xserve to the QuickTake 100.
The iTunes-like UI keeps machines in categories like desktops, notebooks, devices, etc. Click any to reveal a chronological listing of hardware, sorted by folder. Finally, select any piece of hardware (or software) to receive a tremendous amount of information. Processor, RAM, release date, benchmarks and on and on. You can even listen to the startup chime where applicable.
You can make smart folders and even keep track of the hardware you own. I love the built-in timeline, which lets you browse hardware by year of introduction. Best of all, as new hardware is added, Mactracker is updated.
This great piece of freewawre is availabe for the Mac and iPhone/iPod touch [App Store link]. For me, it's indispensable.