Tomorrow is the 30th anniversary of the Mac, so it's a great time to revisit the Macs of the past. You can't run really old Mac software on a modern computer, but there are several ways to emulate older Macs on current Macs and PCs.
System 7 In Your Browser
By far the easiest and most impressive way to check out an old-school Mac is James Friend's PCE.js, a Mac Plus with System 7 emulated in a browser window (click left). Loading up this page threw me right back to ninth grade. Friend's virtual Mac has a bunch of memorable, classic apps, including Microsoft Word and Excel, MacPaint, the Orion astronomy app, Stuffit, HyperCard, and Risk. The only down sides, of course, are that you can't save files and can't add anything to his system. But for a low-impact Mac experience, this simply can't be beat.Emulating a Mac Plus with System 6 via MinivMac
The current state of the art in emulating early Macs is the freeware app, MinivMac which runs on Macs, Windows machines, and Linux. It's easy enough to download and install, although it then needs a few things to work properly: a Mac Plus ROM, a copy of the Mac system software, a blank hard drive image, and whatever apps you want to run.
The ROM (titled "vMac.ROM") is the annoying part, because it's technically illegal to download, although you aren't hurting anyone by downloading a ROM file from a 25-year-old computer.
You can get old versions of the Mac system software from Apple's official site, where you can download System 6.0.3, 6.0.5, 6.0.8, 7.0, or 7.5.3. Mount one of those disks in MinivMac and you'll be in business.
Now go get a 20MB hard drive "blank" from here if you want to install more software. MinivMac has plenty of instructions on how to install old software into your emulator, but it turns out to be a tremendously annoying process because newer computers strip off the file type information that older Macs need to operate. A much easier option is to just download MinivMac-compatible disc images of games from eMaculation's System 6 Hell.
There's a version of MinivMac for jailbroken iPhones, which really bring things full circle, as well as Mini vMac Mac Plus and Mac II emulators for Android devices. (A Mac II on an Android tablet? That's cool.)
More Emulators - System 7 and Above
There are two other major Mac emulators out there, Basilisk II and SheepShaver. Basilisk II is best for emulating System 7 and 8; SheepShaver, for emulating System 8 and 9. However, both are much more complicated to set up and run than minivMac.There are guides, of course, but they can get pretty baroque. eMaculation has pretty deep guides for all of the different emulation permutations, and developer Travis Lawrence did a very patient, thorough guide to setting up SheepShaver from soup to nuts.
For more, check out my thoughts on 27 Years Creating With the Mac.