RTLinux (Real Time Linux), New Mexico Institute of Technology: http://www.rtlinux.org/
RTAI (Real Time Application Interface), Dipartimento di Ingegneria Aerospaziale Politecnico di Milano: http://www.ittc.ukans.edu/kurt/
KURT (The KU Real Time Linux), University of Kansas: http://www.ittc.ukans.edu/kurt/
RED-Linux, University of California, Irvine: http://linux.ece.uci.edu/RED-Linux/index.html