I'm a tool
If you don't monitor your systems you will be toast. My little firm looks out for >1000 servers and systems. Our Icingas et al have around 5000 odd services being monitored, disc space, service/daemon status, AD, DNS, NTP, Equallogics, ESXi, Cisco/Dell/HP/Netgear switch statuses, UPS and many many more including backups. If it's switched on and provides a service, we monitor it.
Nagios/Icinga/Check_MK/NagViz/Nagstamon are my current weapons of choice. I've tried the lot and none comes close. Having said that OpenNMS is looking promising again - I'm probably due another go, the last one was a couple of years ago. If you are a Windows bod then give Spiceworks a go. Its very good.
Netdisco - brilliant if you have a lot of CDP talking network gear. Bit of a pest to get the dependencies sorted out at install. A doddle to use and maint free.
WireShark, nmap - Windows versions are available - compulsory if you go anywhere near a network
Gentoo Linux - mmm compilers n USE flags. Funnily enough my smart new laptop can update itself faster from source code than many Windows update sessions I endure! LibreOffice in an hour and a kernel compile in a fag break. Of more use to the usual mob around here - grab a copy of the System Rescue CD. It will save your bacon one day, even if its simply resetting a forgotten Windows password. That's as close as many will want to get to Linux, but just get it.
Exim, ClamAV and SpamAssassin. Right, that's email sorted. All of that will run on Windows incidentally. Exim can be a bit tricky to set up. Just Goo-Bing-Hoo for a recipe. It can do things with email routing that is amazing. Great to put in front of the usual corporate mail systems thing and rather handy for migrations from one to another system or corp mergers.
Security Onion - it's big and very clever. It will eat disc space if allowed but is a neatly packaged IDS/IPS etc. Expect to spend some time tuning it though.
Graylog2 + ElasticSearch + Logstash. They are all a doddle to install. Logstash can take some configuring. Use nxlog on your Windows servers and send their events using GELF to Graylog. Graylog etc running on my desktop PC (it was top of the line five years ago) for eval ate our external router's Netflow output plus a couple of DCs event and a slack handful of other Win servers plus a mail log and our telephony server logs and Squid logs. Well you get the idea. I run X Windows and KDE on top of that with no slow down. Oh and the query speed is phenomenal. I got up to 40 million records and queries still run in a very few seconds.
FreePBX distro. I can jimmy up a telephone exchange in a VM in under 30 minutes from slapping in the ISO. That includes external trunks and a handful of extensions.
pfSense - it's an amazing firewall, router and VPN concentrator. Works nicely in a VM.
Pretty much everything I've mentioned here is Open Source.