L4 is the successor to the L3 uKernel which was written to prove that uKernel [microkernels] aren't all slow and laggy like Mach. It might be important to note the the GNU Hurd OS is going to be ported to the Pistachio L4 uKernel when it is completed. The major difference between Pistachio and Fiasco is the level of support for which L4 specification.
If you like to futz around with esoteric Operating Systems Fiasco should be on your list. At least check out the demo floppy with the super-cool DoPE windowing system.
An open, cross-platform journaling program.
A scientific plotting package.