Rtai is a realtime extension with a broad variety of services which make realtime programmers' lifes easier. some of them are

choose between fifo and rr scheduling policies on a per task basis.
extended posix apis and message queues, in kernel and user space
semaphores can be typed as: counting, binary and resource
a mailbox implementation
lxrt for hard real time in user space
this package contains the patches for modifying the kernel, and sources for building the kernel modules, module dependent runtime tools and the testsuite.