I've tried to put them in categories, but some of the tools could have been placed in more than one category as they try to provide a solution to several topics.

Operating System

• Ubuntu
• CentOS
• Fedora
• Debian
• FreeBSD
• The Linux series provides some introduction.

Version Control Systems - Source Control Management

• Git
• Mercurial
• Subversion
• Perforce
• Team Foundation Server
• Github
• Bitbucket
• Gogs
• GitLab

Testing Tools and plugins

• Selenium
• PhantomJS
• Cucumber
• Test Kitche

Continuous Integration

• Jenkins - the Jenkins series will cover it.
• Travis-CI
• Appveyor
• Circle CI
• Shippable

Continuous Integration Systems.

Configuration Management

• Ansible - the Ansible series is an introduction
• CFEngine
• Chef
• Docker
• PowerShell DSC
• Puppet
• Rex
• SaltStack
• Sparrowdo

Configuration Management Tools.

Product Management

• Jira
• Basecamp
• Rally
• Pivotal Tracker
• Teamwork
• Changepoint (Daptiv)
• Adobe Workfront
• Version One
• Taiga
• Waffle

Customer Service Desk

• Zendesk
• Zoho desk
• Freshdesk

Scheduling and Incident management

• Pager Duty
• Victor Ops
• OpsGenie

Monitoring

• BMC TrueSight Pulse (was Boundary)
• Data Dog
• Nagios
• Stackify
• Scout
• Dynatrace (was Ruxit)
• Rollbar
• Zabbix
• monit

Security Auditing

• Evident.IO
• Tripwire
• Metasploit
• Nexpose
• Kali
• Nessus
• BeyondSAAS
• Retina
• Core Insight
• Dome9
• Alien Vault
• Alert Logic

Analytics

• New Relic
• Librato

Log Management

• Elastic Search
• Logstash
• Kibana
• Sumologic
• LogEntries
• Splunk
• Loggly

Cloud Hosting

• Amazon AWS
• Digital Ocean
• Linode
• Microsoft Azure
• Google Cloud Services
• OpenStack

Serverless Architectures

• AWS Lambda
• Azure Functions
• Google Cloud functions
• Fission.IO serverless functions on Kubernetes
• IBM Cloud Functions

Virtualization - Containers

• Vagrant
• VirtualBox
• Docker
• Kubernetes
• Buildah for Open Containers (aka. OCI)

Programming languages

• Bash, the Unix/Linux shell is critical if you run Linux machines.
• Python is one of the most frequently used languages by DevOps enginners.
• Groovy is the language used by Jenkins.
• Ruby - there are a few tools written in Ruby and you might need to understand the syntax a bit, but fell out of favor.
• Perl - used to be the favorite language for System Administrators, but it much less popular these days.

Other

• Terraform - Write, Plan, and Create Infrastructure as Code
• Packer - Build Automated Machine Images
• Consul - Service Mesh Made Easy

Inspired by the DevOps Ecosystem by Erik Rogneby.

More from 7 CI/CD tools for sysadmins by Dan Barker