Mini vMac
Mini VMac allows users of newer Macs to run a Macintosh Plus in a window on their computer. http://minivmac.sourceforge.net/doc/start.html
Versions
You need to run a version of Mini vMac compiled to match the ROMs you use. For example, since I'm getting my ROMs from a Mac SE FDHD, I have to use version 24, compiled to match the capabilities of an SE FDHD, and to run on my (Intel) PowerBook Pro. You can get many variations here: http://minivmac.sourceforge.net/doc/var/index.html
CopyRoms
http://minivmac.sourceforge.net/extras/copyroms.html
To run Mini VMac, you need a copy of ROMs from a 680x0 Mac, (from Mac 128, the original, to Mac II). What is difficult to find is how to get a copy of CopyRoms running in an old mac. Instead of providing a program to copy onto a disk, the authors provide a raw disk image with the program on it. Nowhere in the guides do authors tell you how to get the provided image onto a disk so you can put the disk in an old Mac and run CopyRoms. One way to do it is to use dd in linux or OS X. First, you should insert a floppy in a connected drive, then in a terminal (shell), you should use the mount command to find out what the name of the floppy device is. In my case, I type mount, and I see many lines, one line being
/dev/disk1 on /Volumes/Unlabeled (local, nodev, nosuid)
Since my floppy was called Unlabeled, I know this is it, so I use dd to copy the CopyRoms disk image to it. But to access a drive at the base level, it needs to be unmounted (else a "device busy" error), so use Disk Utility to unmount the volume, but don't eject it. Then go back to terminal and enter the following command, being sure to substitute your floppy device name for the last argument (mine was /dev/disk1, but yours may be different).
dd bs=2x80x18b < /path-to/copyroms-1.1.0.dsk > ( your floppy device name)
WARNING! If you put the wrong device at the end of that command, you could mess up your hard drive forever. Make sure you don't type the wrong device. disk1 in your case might be a hard drive or anything, so make sure to use the mount command first to find the right device. It would be best, instead of typing it, to copy and paste from the output of mount, and still double-check.
I had a problem, CopyRoms was not making a non-zero rom file in my SE FDHD. I realized the problem was that when I used dd to copy the image to the floppy, it made a floppy with only a bit more capacity than the image, so there was no room for CopyRoms to place the rom file onto the disk. When I copied CopyRoms to the hard drive in my SE, it worked. Another workaround would be, once you've made the floppy, copy the program onto your hard drive, reformat the floppy to full size, then copy CopyRoms back onto the floppy. Reformatting works in Snow Leopard, but Jaguar can't format floppies.
System Software
The authors of Mini vMac suggest getting System 6.0.8 software from Apple: http://www.info.apple.com/support/oldersoftwarelist.html You'll need Stuffit Expander to expand the SEA archives. Stuffit Expander is free from Smith Micro: http://www.stuffit.com/mac-expander.html
Alternatives
Infinite Mac offers various Mac OS 10 and NeXT computers that run in a web browser. Now you can have NeXT on your phone, so you can frustrate yourself with awkward awk functions.
