Embedded Configurable operating system (spelled eCos) is an embedded RealTimeOperatingSystem. Originally from Cygnus, then RedHat [http://sources.redhat.com/ecos/ ]
It is free, covered by something GPLish. It is much lighter than embedded linux, even UcLinux. Comparable to RtLinux. It seems to mainly be eating into VxWorks' area, but I'm not sure.
See EmbeddedSystem.