Monit is another standard program, but I think these these things being compared are really not similar. Aren't upstart and systemd replacements for init.d runlevel scripts? While God & Monit make sure programs are running. I suppose there is overlap.
There's no clear delineation here. Upstart and Systemd are init.d replacements, both of which can make sure that programs are up and running, and incorporate some basic process monitoring. Runit is similar, and can replace init.d, but will happily run as just another process. God can't replace init.d, but it can easily handle the daemonize-and-keep-running functionality that's at the core of what we want from all this stuff. In addition, God can do some health checking, for things like memory and CPU consumption. IIRC Monit doesn't actually start other programs under itself, but it can be configured to watch running programs, and kick them if necessary.
If you have a clean and useful taxonomy for this stuff, I'd be interested in hearing it.
Monit will startup a program if it's not running according to it's pid file. In other words if the pid file doesn't exist it will start the program. I use it in deploys by killing a program and letting Monit start it again with the new version.
systemd encompasses process monitoring functionality by design, which is a strength imho, though it is subject to the "all your eggs in one basket" criticism.
