Pcemu is a simple IBM PC emulator, that emulates an i8086 CPU, along with text mode VGA hardware. The first version was written by David Hedley, as part of his Final Year Project. You can read his report, and associated documentation here.
Currently it can be downloaded on sourceforge.
While restricted to text mode programs, it can run:
MSDOS 5.0 MSDOS 6.2 WordPerfect 5.1 Borland C++ 2.0 Turbo Debugger 2.51 Turbo Assembler 2.51 BBCBasic 4.61 MSDOS QBasic MSDOS GWBASIC Virtually all program that came with MSDOS 5 Hitchhiker's Guide to the Galaxy PC Magazine's ANSI.COM SemWare's QEdit 2.1 Norton Utils 4.50 Advanced Edition Norton Utils 6.0 Xtree Professional 1.1 PowerMeter Utils Autoroute (ancient version) Minitab 8.0 Microsoft Diagnostics
pcemu is relativly portable, and can run on Sun & i386 machines. I've no luck with the DEC Alpha, but I haven't looked to much into it. Also I've done a super basic port to Win32/Win64, with Visual C++ 5.0 and higher.