The WDC W65C816 microprocessor is a 16-bit enhancement to the 8-bit 6502 that is fully backward compatible. It formed the core of the Apple IIgs computer, as well as the Super Nintendo gaming platform. It currently finds its greatest use in embedded systems, and as homebrew upgrades to traditionally 8-bit platforms like the Apple II, Atari 8-bit, and Commodore 8-bit computer lines.
The 65816 also happens to make a fantastic homebrew hacker friendly processor due in large part to its very simple bus interface. However, when building homebrew computers, it's often a good idea to build a software emulation (or concept design) first. However, all the 65816 emulators available tend to not be portable. KEGS has a 65816 emulator that is nearly impossible to isolate from the Apple IIgs core logic, for example.
For this reason, I've attempted to box lib65816 sufficiently well so that it can be re-used in any number of different projects. It's not the best emulator, by far, but it is portable and convenient to work with.
As it is still a work in progress, there is currently no convenient tarball download. However, you may use darcs to download the latest build, like so:
Note that this is just a CPU emulator -- it still requires additional software to make a computer emulator. For example, see the Kestrel 2 emulator, which uses lib65816 as its CPU emulation core.