systemd - systemd is a replacement for the init daemon for Linux (either System V or BSD-style).

sysvinit - Savannah is a central point for development, distribution and maintenance of free software, both GNU and non-GNU.

runit - runit is a cross-platform Unix init scheme with service supervision, a replacement for sysvinit...

finit - Finit is a simple alternative to SysV init and systemd, reverse engineered from the EeePC fastinit ten years ago by Claudio Matsuoka — “gaps filled with frog DNA …” Finit supports runlevels, process monitoring, and starting services on demand using …

s6 - s6 is a small suite of programs for UNIX, designed for process supervision. It can be used as an init system, or as separate supervision components.

Supervisor - Supervisor is a client/server system that allows its users to monitor and control a number of...