The Apache Software Foundation maintains hundreds of open-source projects. If you’re asking about the most useful and widely adopted Apache products, these are among the most influential:

Project	What it’s used for
Apache HTTP Server	One of the world’s most popular web servers for hosting websites and web applications.
Apache Kafka	Real-time data streaming, event processing, and data pipelines. Used heavily by large-scale applications.
Apache Spark	Fast big-data processing, analytics, machine learning, and ETL workloads.
Apache Hadoop	Distributed storage and processing of large datasets across clusters.
Apache Airflow	Scheduling and managing data pipelines and automated workflows.
Apache Cassandra	Highly scalable database designed for high availability and massive workloads.
Apache Flink	Real-time stream processing and event-driven applications.
Apache Tomcat	Running Java web applications and APIs.
Apache Maven	Dependency management and project builds for Java development.
Apache Superset	Data visualization, dashboards, and business analytics.
Apache NiFi	Moving, transforming, and managing data between systems.
Apache Lucene	Full-text search capabilities used by many applications.
Apache Solr	Search and indexing for websites and enterprise applications.

By category

For web development

• Apache HTTP Server
• Apache Tomcat

For data engineering

• Apache Kafka
• Apache Airflow
• Apache NiFi

For big data and analytics

• Apache Spark
• Apache Flink
• Apache Hadoop

For databases

• Apache Cassandra

For search

• Apache Lucene
• Apache Solr

If you’re learning in 2026, the Apache projects with the strongest demand in modern cloud and data engineering roles are generally:

1. Apache Kafka
2. Apache Spark
3. Apache Airflow
4. Apache Flink
5. Apache Cassandra