Here is a list of various realtime operating systems. Each name is linked to some further information.Since these kind of surveys are not our main job, the list is probably incomplete and might not be up to date either (good things go bad, bad things go out of business). 

ThreadX

AMX

xPC Target

MQX

VxWorks

RTXC

C Executive

MicroC/OS-II

EUROS

Windows CE

OS-9

Embedded DOS 6-XL

RT-Target 32

OnCoreOS

QNX-Neutrino

eCos

PXROS

RealtimeCraft

LynxOS

SuperTask! Multitasking RTOS Development Suite

TronTask! Multitasking RTOS Development Suite

ARTOS

eXecutive

ChorusOS

EOS

Granada

Hard Hat Linux

Inferno

PDOS

pF/x

pSOS

RTLinux

RTEMS

RTMX O/S

RxDOS

SMX

SwiftOS

velOSity

VRTX

XOS/IA-32