L4 is under active development, will support SMP soon, shows promise as a high-performance microkernel, and has implementations on x86, ARM, MIPS, and Alpha, and will be ported to the PowerPC and Motorolla 68k processors in the future. This page contains a collection of links presenting on the mailing lists over the past year or so. It provides a great jumping point for subjects relating to the Hurd. The traffic is mild compared to say, the linux kernel, and are the main focal points of the Hurd development. You can submit bugs using Debian bug tracking system too.
Related packages are hurd and gnumach .
At this point, you should have a good basis for using the Hurd. T he first thing to understand is MIG, the mach interface generator. Since the IPC interface in Mach is language independent and fairy complex, you use MIG to automatically generate the C procedures to pack and send, or receive and unpack the IPC messages used to communicate between processes.