Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
QEMU is a fast processor emulator. Using dynamic translation it achieves a reasonable speed while being easy to port to new host CPUs. In its user mode emulation mode, it can launch Linux processes compiled for one CPU on another CPU. Linux system calls are converted because of endianness and 32/64 bit mismatches. In its full system emulation mode, it emulates a full system, including a processor and various peripherials.
LibDsk is a library that attempts to create uniform functions for accessing floppy drives, raw "dd" disk images, and disk image files in various emulator formats. Its intended use is for emulator authors; it also includes some sample tools to read sectors from discs in CP/M, DOS, and Acorn formats. There is special-case code for direct access to the floppy controller under Linux, and to access the floppy driver under Windows. Java (JNI) bindings are included.
XRoar is a cross-platform emulator for the Dragon 32, Dragon 64, and Tandy CoCo computers, all originally released in the early '80s. The emulator runs under Unix-like operating systems (including Mac OS X) and Windows, with older versions available for handhelds like the Nintendo DS and GP32.