Why?
I originally ported this so that I could debug a program using OS X’s command line tools. I would use the emulator to run the program and create a dump file full of verbose output, save that on the shared Unix partition, and then use diff, sed, grep and friends to search through the dump file.
Some people use it to run programs that are too old to run under Mac OS 9 (and thus, won’t run under Classic).
With Intel Macs no longer supporting Classic, and a lot of legacy programs, that need isn’t going to go away.
About my Basilisk II port
Emulates: Mac OS 7–8.1 running on a 68020-68040
CD support: HFS/ISO9660
Floppy: No
Graphics: Fullscreen and windowed. Allows emulated OS to change res. & depth Networking: EtherNet only(AppleTalk status unknown)
Printing: No
Serial: Untested
Sound: 16bit only
Other ports:

1. •Gwenole’s JIT build
2. •Gwenole’s SheepShaver

Useful links:

1. •Emulation.net
2. •Low End Mac article

Bugs
Users have encountered a few problems with 10.4, G5s, and Intel Macs. I have not been able to reproduce these, so if you are able to help (with detailed bug reports, Console.app output, preferences/ROM/boot disk files)
I would appreciate it.
I have been able to reproduce the Intel 68040 crash,
disappearing mouse in fullscreen mode,
and problems in 256 colour.
I am slowly working on these.
V16 Classic
May 2004
“Classic” here means this should be able to emulate a MacClassic (with appropriate ROM and MacOS <= 7.5)
V19b Universal
August 2006
Universal application. Sound (and paste) support
V19b
August 2006
PPC application. Sound (and paste) support.
V17
January 2006
PPC application. Working EtherNet, some 10.4 fixes.