Windows: Xming + gnuwin32 + mingw-w64 + MSYS

	Xming provides an Xserver
	gnuwin32 provides glibc (which gcc should be made to link to) and coreutils, etc.
	msys provides make and bash

Linux: linux-libre + x.org + glibc + coreutils + ash

Embedded Linux: x.org + uclibc + busybox

	Busybox provides ash

Android: An android kernel + uclibc? + libhybris + busybox

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For developer's portability:


NATIVE:
If you are writing a C program, try to write in ANSI C.
If you're writing a shell script, try to write in ash.

BYTECODE:
Parrot also allows compiling a program to bytecode and running on a different system.