1. CI/CD (Continuous Integration/Continuous Deployment)
Jenkins
A widely used open-source automation server that supports building, deploying, and automating across various environments.
GitLab CI
Integrated with GitLab, it offers CI/CD functionality for automating testing and deployment.
Argo CD
A declarative, GitOps continuous delivery tool for Kubernetes applications.

2. Containerization
Docker
The most popular open-source containerization platform, allowing you to package and isolate applications.
Podman
An alternative to Docker, offering rootless containers and daemonless architecture.

3. Container Orchestration
Kubernetes
The leading container orchestration platform for automating the deployment, scaling, and management of containerized applications.
Nomad
A simpler alternative to Kubernetes for orchestrating containers and non-containerized applications.

4. Service Mesh
Istio
A service mesh to connect, secure, and observe services, providing traffic management, security policies, and observability.
Linkerd
A lightweight, open-source service mesh that enhances observability and security between microservices.

5. Monitoring and Logging
Prometheus
A widely adopted open-source monitoring and alerting toolkit designed for reliability and scalability.
Grafana
A platform for monitoring and observability, often used in combination with Prometheus for visualization of metrics.
ELK Stack (Elasticsearch, Logstash, Kibana)
A set of tools for logging, searching, and visualizing logs and metrics from different sources.
Jaeger
An open-source distributed tracing system for monitoring and troubleshooting transactions in complex microservices architectures.

6. Security and Compliance
Anchore
A tool for scanning and auditing container images to ensure they meet security and compliance policies.
Trivy
A simple and comprehensive vulnerability scanner for containers, Kubernetes, and other artifacts.

7. API Gateway
Kong
An open-source API gateway and platform for managing, securing, and analyzing APIs and microservices traffic.
NGINX
Widely used as an API gateway, load balancer, and reverse proxy for microservices.

8. Infrastructure as Code (IaC)
Terraform
An open-source tool for building, changing, and versioning infrastructure safely and efficiently.
Ansible
An open-source automation tool used for configuration management, application deployment, and task automation.
Pulumi
An open-source IaC tool that supports multiple languages and integrates well with cloud-native technologies.

9. Configuration Management
Helm
A Kubernetes package manager that makes it easy to manage, update, and roll back Kubernetes applications.
Consul
A tool for discovering and configuring services in your infrastructure, often used for microservices.

10.Testing
Postman
Although not entirely open-source, Postman has a free version and is commonly used for API testing.
K6
An open-source load testing tool built for testing the performance of APIs, microservices, and web applications.

11. Distributed Message Queuing
Apache Kafka
A distributed event streaming platform used to build real-time data pipelines and streaming applications.
RabbitMQ
A message broker that supports multiple messaging protocols and is commonly used for building distributed systems.