My List:

Oxidized as a replacement for RANCID
Telegraf + InfluxDB = Tons of Grafana Dashboards
(Open Source Slack Alternative)
Ansible or Python Knowledge with Paramiko or netmiko for network automation.

BGP:

FRRouting - Mimics Cisco CLI
BIRD - Programming style config format.
Exabgp - Mostly used for API driven applications, monitoring with heartbeat scripts.
(many others)

DDoS detection and/or filtering:

Fastnetmon - Supports many methods for packet processing.
Ddosdetector (IPv4 Only) - Uses netmap for packet processing.

Top Talkers + Other Creativeness (like fib compressing, or route optimization):

pmacct - sflow/netflow combined with BGP, and a database backend

Servers:

Sensu or LibreNMS for Nagios type monitoring.

Diagnostics:

MTR - ...and knowing how to interpret it's output.

-Ryan