runit is a cross-platform Unix init scheme with service supervision; a replacement for sysvinit and other init schemes. It runs on GNU/Linux, *BSD, Mac OS X, and Solaris, and can easily be adapted to other Unix operating systems. runit implements a simple three-stage concept. Stage 1 performs the system's one-time initialization tasks. Stage 2 starts the system's uptime services (via the runsvdir program). Stage 3 handles the tasks necessary to shutdown and halt or reboot.
eINIT is an alternative init system. It's quite fast, since it can potentially be used without the help of any scripting at all, but it is still flexible and extremely modular. The focus is on speed and parallelisation, mostly with embedded devices and low-downtime servers in mind, and benchmarks do suggest that it's doing a fairly good job at that. It compiles cleanly (and should thus work, provided someone writes appropriate modules) on Linux, FreeBSD, and Darwin/Mac OS X.
pmtr starts your application daemons (not the system daemons) at system boot and lets you dynamically add, remove, or edit jobs at runtime. What makes pmtr different from sysvinit and similar systems is that all your jobs are defined in one configuration file, and the syntax is friendly.