For the uninformed (me, previously ):

Basilisk II is an open source software emulator which emulates the 680x0-based Apple Macintosh computer on a variety of operating systems, including BeOS, Linux, AmigaOS, Windows NT, Mac OS X and even on the Sony PSP.

The last version of Mac OS that can be run within Basilisk II is Mac OS 8.1, the last 680x0-compatible version. Newer Mac OS versions aren't compatible with Basilisk II because those newer versions require a PowerPC-based processor, which can't be emulated by the program. To run such operating systems, emulators that can emulate PowerPC-based processors, like SheepShaver or PearPC, are needed. Alpha versions were available from January 1999 with the first non-alpha release in October of the same year.

Since the new Macs that use the new Intel chips cannot run in Classic mode Basilisk II is valuable as an alternative for users who wish to run programs that would previously have to be run in Classic mode.

[quoted from wikipedia]