Sheepshaver is an emulator (technically, a run-time environment) for Mac OS 7.5.3 through 9.0.4 coded by Christian Bauer and ported to Mac OS X by Ronald P. Regensburg. It requires original system files (i.e., those found on an installation CD) and a significant amount of tweaking to get working, but once it's up it works quite well. You'll be plunged into nostalgia when you see the old loading screen on your high-tech LCD screen.
A major drawback of SheepShaver is that it does not support Quicktime past a certain version. Because of this, a number of applications that might otherwise run on supported systems cannot be opened in SheepShaver. Another drawback is the difficulty of setting up your virtual system. Even with the untold hours I have spent setting it up (I like my classic apps!), there are still difficulties and bugs I have simply had to work around or ignore. All that said, it's the only emulator for these systems I'm aware of, and it works very well for about half the Classic apps I've tried it out with. For that alone I love it!
Sam's Protip: System files and ROMs are available on various parts of the web. Obviously Apple does not sell the systems itself anymore, classifying them as abandonware in my opinion. Still, they remain officially Apple's, which is why they're not included with the SheepShaver download.
