[mfincham]

RT from BestPractical https://www.bestpractical.com/rt/. Extremely extensible and robust, can be used for all kinds of things beyond just ticket tracking (change control approval workflows, knowledge bases, asset tracking, CRM and automation plugins). The king of ticket trackers, could be at the heart of any organisation.

Prosody https://prosody.im/. Open source XMPP server with support for MUCs and all the usual XMPP stuff. Best way for teams to communicate. Extend it with plugins for your NMS and so on to get alerts in realtime.

Nagios https://www.nagios.org/. Solid (if a little fiddly) monitoring / alerting solution. Works best if you write your own plugins and use check_by_ssh instead of NRPE.

bcfg2 http://bcfg2.org/. Configuration management that makes sense. Tidy and well defined "model" for how configuration should be managed, sensible defaults and a bunch of useful powerful features.

I could go on but there are some of my all-time favourites.