Remap Keyboard is described as 'lets you modify your Mac keyboard layout by changing the behavior of each key to work like another. You can apply mapping to all your keyboards or just the selected keyboard (e.g. only the MacBook keyboard or only the USB keyboard)' and is an app in the OS & Utilities category. There are four alternatives to Remap Keyboard for Linux, Mac, Windows, X11 and BSD. The best alternative is SharpKeys, which is both free and Open Source. Other great apps like Remap Keyboard are Karabiner Elements (Free, Open Source), xmodmap (Free, Open Source) and Kinto (Free, Open Source).
SharpKeys is a utility that manages a Registry key that allows Windows to remap one key to any other key. Included in the application is a list of common keyboard keys and a Type Key feature to automatically recognize most keyboard keys. It was developed in C# using .NET v2.
Karabiner, previously called KeyRemap4MacBook, is a very powerful keyboard remapper for Mac OS X. In addition to simple key remapping, it has special remapping modes like Emacs-mode, SandS-mode (Space and Shift).
The xmodmap program is used to edit and display the keyboard modifier map and keymap table that are used by client applications to convert event keycodes into keysyms. It is a part of the X.Org project.