SheepShaver provides a MacOS environment running under Mac OS X (the only one under Intel-based Macs).
Features (from official site):
* Runs MacOS 7.5.2 thru 9.0.4.
* Color video display
* CD quality sound output
* Access to floppy disks, CD-ROMs and HFS(+) partitions on hard disks
* Easy file exchange with the host OS via a “Host Directory Tree” icon on the Mac desktop
* Internet and LAN networking via Ethernet
* Serial drivers
* SCSI Manager (old-style) emulation