Unix

Concurrent Computer Corp.
Makes PowerMAX OS: fully preemptive, multi-threaded, Unix-based RTOS. Leading provider of concurrent real-time multiprocessor OSs, high-performance real-time computer systems, solutions, software for business and government. Focus: strategic business areas including: simulation; data acquisition; instrumentation and process control; interactive real-time (includes wagering, gaming, video-on-demand); vibration control.
http://www.ccur.com/

Maruti
Goal: create an environment for developing and deploying critical applications with hard real-time constraints in a reactive environment. Such applications must be able to execute on a platform of distributed and heterogeneous resources, and operate continuously in the presence of faults.
http://www.cs.umd.edu/projects/maruti/

RTMX, Inc.
Makes RTMX O/S: IEEE POSIX realtime extensions to OpenBSD, with vendor specific ports for embedded, dedicated systems, application specific Internet/intranet servers. Many fine features. North Carolina, USA.
http://www.rtmx.com/