AppScale - Open source cloud software with Google App Engine compatibility.
Archipel - Manage and supervise virtual machines using Libvirt.
CloudStack - Cloud computing software for creating, managing, and deploying infrastructure cloud services.
Cobbler - Cobbler is a Linux installation server that allows for rapid setup of network installation environments.
Eucalyptus - Open source private cloud software with AWS compatibility.
Mesos - Develop and run resource-efficient distributed systems.
OpenNebula - An user-driven cloud management platform for sysadmins and devops.
Openshift Origin - Open source upstream of OpenShift, the next generation application hosting platform developed by Red Hat.
OpenStack - Open source software for building private and public clouds.
The Foreman - Foreman is a complete lifecycle management tool for physical and virtual servers. FOSS.
Tsuru - Tsuru is an extensible and open source Platform as a Service software.
Terraform - Terraform allows you to practice infrastructure as code and is commonly used for AWS/GCE.
Cloud Orchestration
BOSH - IaaS orchestration platform originally written for deploying and managing Cloud Foundry PaaS, but also useful for general purpose distributed systems.
Ansible - Contains modules for controlling many types of cloud resources.
Cloudify - Open source TOSCA-based cloud orchestration software platform written in Python and YAML.
consul - It is a tool for discovering and configuring services in your infrastructure.
doozerd - Doozer is a highly-available, completely consistent store for small amounts of extremely important data.
etcd - A highly-available key value store for shared configuration and service discovery.
Juju - Cloud orchestration tool which manages services as charms, YAML configuration and deployment script bundles.
MCollective - Ruby framework to manage server orchestration, developed by Puppet labs.
Overcast - Deploy VMs across different cloud providers, and run commands and scripts across any or all of them in parallel via SSH.
StackStorm - Event Driven Operations and ChatOps platform for infrastructure management. Written in Python.
zookeeper - ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services.
Cloud Storage
git-annex assistant - A synchronised folder on each of your OSX and Linux computers, Android devices, removable drives, NAS appliances, and cloud services.
nextCloud - Provides access to your files via the web
ownCloud - Provides universal access to your files via the web, your computer or your mobile devices.
Seafile - Another Open Source Cloud Storage solution.
SparkleShare - Provides cloud storage and file synchronization services. By default, it uses Git as a storage backend.
Swift - A highly available, distributed, eventually consistent object/blob store.
Syncthing - Open Source system for private, encrypted and authenticated distribution of data.
Code Review
Web Based collaborative code review system.
Gerrit - Based on the Git version control, it facilitates software developers to review modifications to the source code and approve or reject those changes.
Phabricator - Code review tool build by facebook and used by WikiMedia, FB, dropbox etc. Comes with an integrated wiki, bug tracker, VC integration and a CLI tool called arcanist.
Clusto - Helps you keep track of your inventory, where it is, how it’s connected, and provides an abstracted interface for interacting with the elements of the infrastructure.
Collins - At Tumblr, it’s the infrastructure source of truth and knowledge.
Courier IMAP/POP3 - Fast, scalable, enterprise IMAP and POP3 server.
Cyrus IMAP/POP3 - Intended to be run on sealed servers, where normal users are not permitted to log in.
Dovecot - IMAP and POP3 server written primarily with security in mind.
Qpopper - One of the oldest and most popular server implementations of POP3.
Mail Transfer Agents (SMTP servers).
Exim - Message transfer agent (MTA) developed at the University of Cambridge.
Haraka - A high-performance, pluginable SMTP server written in JavaScript.
MailCatcher - Ruby gem that deploys a simply SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development.
Maildrop - Open Source disposable email SMTP server, also useful for development.
OpenSMTPD - Secure SMTP server implementation from the OpenBSD project.
Postfix - Fast, easy to administer, and secure Sendmail replacement.
Collectl - High precision system performance metrics collecting tool.
dashing - No Longer Maintained - Ruby gem that allows for rapid statistical dashboard development. An all HTML5 approach allows for big screen displays in data centers or conference rooms.
Smashing - Ruby gem that allows for rapid statistical dashboard development. An all HTML5 approach allows for big screen displays in data centers or conference rooms. Fork of Dashing.
Diamond - Python based statistic collection daemon.
Facette - Time series data visualization and graphing software written in Go.
Freeboard - A damn-sexy front-end real-time dashboard. Transforms raw JSON into delicious UI.
Ganglia - High performance, scalable RRD based monitoring for grids and/or clusters of servers. Compatible with Graphite using a single collection process.
Grafana - A Graphite & InfluxDB Dashboard and Graph Editor.
Galera - Galera Cluster for MySQL is an easy-to-use high-availability solution with high system up-time, no data loss, and scalability for future growth.
PostgreSQL - Object-relational database management system (ORDBMS).
PostgreSQL-XL - Scalable Open Source PostgreSQL-based database cluster.
SQLite - Library that implements a self-contained, serverless, zero-configuration, transactional SQL DBS.
Security
Security tools.
Blackbox - Safely store secrets in Git/Mercurial. Provides tooling to automatically encrypt secrets like passwords.
Denyhosts - Thwart SSH dictionary based attacks and brute force attacks.
Fail2Ban - Scans log files and takes action on IPs that show malicious behavior.
fwknop - Protects ports via Single Packet Authorization in your firewall.
Glastopf - A low-interaction web application honeypot to emulate vulnerabilities and gather attack data.
Kippo - A medium-interaction SSH honeypot, mostly used as a standalone SSH daemon with a configurable Filesystem sandbox.
OSSEC - OSSEC is a HIDS that performs log analysis, FIM, rootkit detection, and much more.
OSQuery - Query your servers status and info using a SQL like interface.
pfSense - Firewall and Router FreeBSD distribution.
Snort - Snort is a free and open source network intrusion prevention system (NIPS) and network intrusion detection system (NIDS) created by Martin Roesch in 1998.
SpamAssassin - A powerful and popular email spam filter employing a variety of detection technique.
BounCA - BounCA is a personal SSL / Certificate Authority Key management tool. Create self-signed SSL certificates via your browser. (Source Code) ApachePython
Service Discovery
Consul - Consul is a tool for service discovery, monitoring and configuration.
Doozerd - Doozer is a highly-available, completely consistent store for small amounts of extremely important data.
ZooKeeper - ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services.
Software Containers
Operating system–level virtualization.
Bitnami - Produces open source installers or software packages for web applications and development stacks as well as virtual appliances.
Docker - Open platform for developers and sysadmins to build, ship, and run distributed applications.
LXC - Userspace interface for the Linux kernel containment features.
[Learn Cisco Network Administration in a Month of Lunches] (https://www.manning.com/books/learn-cisco-network-administration-in-a-month-of-lunches) - A tutorial designed for sysadmins who need to learn how to administer Cisco switches and routers.
The Practice of System and Network Administration - The first and second editions describes the best practices of system and network administration, independent of specific platforms or technologies.
Securing DevOps - A book on Security techniques for DevOps that reviews state of the art practices used in securing web applications and their infrastructure.
Newsletters
Servers for Hackers - Newsletter for programmers who find themselves needing to know their way around a server.
DevOpsLinks - A community of DevOps, SysAdmin & Developers with a weekly newsletter and a team chat.
Repositories
Debian-based distributions.
Dotdeb - Repository with LAMP updated packages for Debian.
RPM-based distributions.
ElRepo - Community Repo for Enterprise Linux (RHEL, CentOS, etc).
EPEL - Repository for RHEL and compatibles (CentOS, Scientific Linux).
Remi - Repository with LAMP updated packages for RHEL/Centos/Fedora.