JPC - Computer Virtualization in Java - a PC emulator in Java. Strange.
emulation
PC Systems on the Mac
Who doesn't feel like paying Microsoft for Virtual PC and isn't exactly thrilled by the other well-known alternatives, might want to check out a less known one: iEmulator PC Emulator for MAC OS X is an emulator for the Mac based on QEmu. The special feature: not only the normal 32-bit Intel chips can be emulated, but also the 64-bit ones, as well as Sparc, ARM, and PPC. Pretty cool, that thing. And for those who find even the 25 dollars for iEmulator too much (or are hardcore OSS users), there's QEmuX - a free graphical interface for QEmu on the Mac.
To take it for a test drive, you can get suitable pre-prepared images from FreeOSZoo.
In initial tests, I had the usual problem: the keyboard layout just doesn't fit. You can only get something similar to a German keyboard layout, it doesn't fit exactly - the umlauts are off, some special characters are wrong, the whole thing is quite rough in that regard. Not that Virtual PC is any better: I could never properly enter the angle brackets and the pipe symbol under Virtual PC (which is pretty stupid for programmers).
Kallisys | Newton | Einstein Project
The Einstein Project is an emulator for the Apple Newton hardware. While this doesn't make your own Mac any smaller (and thus isn't a replacement for the PDA), you can at least play around with one of the most interesting PDA operating systems. Found in Rainer Joswig's Lispnews.
PearPC - About - Emulator for PPC hardware on non-PPC systems
HP-12C for Mac OS X - Another emulation of an HP calculator
UADE - Unix Amiga Delitracker Emulator - an amiga music file player for unix - UAE-based player for old Amiga sound formats