RuckusRD is a drop-in replacement for mkinitrd that's intended for use on embedded systems or removable media (e.g. live CDs), but is also perfectly usable on normal desktop Linux systems. Its main strength is its non-memory-retentive read-write root filesystem support, which combines a read-only SquashFS image with a RAM overlay. This utility is part of the Source Ruckus project.
Source Ruckus Linux is an embeddable, robust Linux meta-distribution. It's a collection of build tools, source packages, scripts, and documentation all ready to be built into a Linux distribution. It can be used out-of-the-box, but its real strength lies in being used as a base for your own distribution. In other words, it exists purely to be forked, although it can be used as-is.