Try Basilisk II; much easier to set up than Fusion. Both run only as a Mac 68000-compatible OS 8.1 (which runs only IE or Netscape versions 4.0). Can be easily run as an application within Windows. Supposedly, Fusion's author "is working" on a PowerPC-compatible version.
Then there's Darwin, which can be downloaded in ISO format, directly from Apple. It's the OS X kernal.