RealTime Application Interface (aka RTAI) - for GNU/Linux - lets you write applications with strict timing constraints for your favourite operating system. Like Linux itself this software is a community effort. A patch to the Linux kernel which introduces a hardware abstraction layer. A broad variety of services which make realtime programmers' lifes easier. RTAI supports several architectures:

RTLinux (or Real-Time Linux) is an extension of Linux to a real-time operating system. RTLinux was made available as a community supported free version, called RTLinux Free, as well as a commercial version, called RTLinux Pro. Wind River Systems acquired FSMLabs technology in February 2007 and now makes the technology available as Wind River Real-Time Core for Wind River Linux (from Wikipedia).

Xenomai is a real-time development framework cooperating with the Linux kernel, in order to provide a pervasive, interface-agnostic, hard real-time support to user-space applications, seamlessly integrated into the GNU/Linux environment. The Xenomai project has been launched in August 2001. It has merged in 2003 with the RTAI project to produce an industrial-grade real-time Free Software platform for GNU/Linux called RTAI/fusion, on top of Xenomai's abstract RTOS core. Eventually, the RTAI/fusion effort became independent from RTAI in 2005 as the xenomai project.