Operating systems
- Ubuntu
Programming/scripting languages
- Go
- Python/Ruby
Configuration management
- Chef
- Ansible
Monitoring/graphing/logging/searching
- Sensu
- Graphite
- Logstash
- ElasticSearch
Load balancer/Web server
- HAProxy
- Nginx
Relational databases
- MySQL
- PostgreSQL
Non-relational distributed databases
- Riak
- Cassandra
Service discovery
- etcd
- consul
Virtualization
- KVM
- Vagrant
- Docker
Software defined networking (SDN)
- Open vSwitch
IaaS
- OpenStack
PaaS
- CloudFoundry
This should keep most people in the industry busy for a while ;-)