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.
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.
spim is a self-contained software simulator for running R2000/R3000 assembly language programs. It reads and can immediately execute files containing assembly language code. spim also provides a debugger and simple set of operating system services. spim provides both a simple, textual interface and a fancier, graphical interface. The package includes complete source code and documentation.
OpenVPS is a set of software built on top of the Linux VServer aimed specifically at Web Hosting. It is not another set of kernel patches, but a set of scripts to create virtual servers, collect resource utilization information, and provide an interface to the customer as well as the administrator.
The Intel Software Development Emulator is a functional emulator for new and upcoming instruction set extensions. The emulator is based on the Pin dynamic binary instrumentation system (and XED). It emulates the new instructions in the SSE4, AES, PCLMULQDQ, and RTM, BMI1, and BMI2 Intel AVX and AVX2 instruction set extensions. This allows developers to gain familiarity with Intel's upcoming instruction set extensions.