Shoebill is described as '- a Macintosh II emulator that runs A/UX (and A/UX only)'. There are five alternatives to Shoebill for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is AppleWin, which is both free and Open Source. Other great apps like Shoebill are Basilisk II (Free, Open Source), Mini vMac (Free, Open Source), PCE (Free, Open Source) and Sheepshaver (Free, Open Source).
Basilisk II is an Open Source 68k Macintosh emulator. That is, it allows you to run 68k MacOS software on your computer, even if you are using a different operating system. However, you still need a copy of MacOS and a Macintosh ROM image to use Basilisk II.
The Mini vMac emulator collection allows modern computers to run software made for early Macintosh computers, the computers that Apple sold from 1984 to 1996 based upon Motorola's 680x0 microprocessors. The first member of this collection emulates the Macintosh Plus.
PCE is a collection of microcomputer emulators. At the moment it contains three emulators: PCE/atarist is an Atari ST emulator. It emulates the various ST and Mega ST models. PCE/ibmpc is an IBM PC 5150 emulator. It emulates an IBM PC 5150 or an IBM PC/XT 5160.