Mac OS X/iOS: While System Profiler is handy for checking out your hardware, but it doesn't always tell you everything (and it isn't the most user friendly). Free app Mactracker gives you all the info you need about your computer.
Whether you're looking to upgrade your RAM, want to know how high you could push an external display's resolution, or just what versions of Mac OS your old computer supports, Mactracker will give you all the info you need to know about your computer. It's a bit more user-friendly than System Profiler, and gives you some other interesting tidbits System Profiler won't. Plus, you can get information about any computer — not just the one you're running the program on. You can search through any Apple product ever created, from laptops to iPods to keyboards to the Newton. It also detects which model computer you have, so you don't need to ask yourself "when did I buy this?" It's especially useful for the old machine you've pulled out to upgrade or repurpose.
The only thing that's a little annoying is that it's more of a product guide than a real system info tool like previously mentioned Speccy on Windows. It won't tell you the clock speed of your processor or the size of your hard drive, only what options were available on that Mac when it was sold. Still, the ability to give you more hard-to-find information like what resolution your laptop supports on an external monitor, and to be able to do it for any Mac in history, is pretty darn useful.