Some weeks ago I "Show HNed" my project ping.gg[0] and some people asked to have it open sourced. I know it’s unlikely that those guys will see this again now, but I can not comment on the thread anymore so there you go anyway[1].
This is the core of the system and enough to build your own private ping.gg. It's obviously really simple and limited. Adding support for HTTP status checks will require some mayor refactoring.
[0] https://news.ycombinator.com/item?id=8774257
[1] https://github.com/pinggg/pingd