​Apache Kafka, Hue, Hadoop YARN, and HBase—are open source. Here’s a brief overview of each tool and its open-source nature:

1. Apache Kafka: This is a distributed event streaming platform capable of handling trillions of events a day. Initially conceived as a messaging queue, Kafka is based on an abstraction of a distributed commit log. It is designed to allow for high-throughput, low-latency processing of real-time data feeds. Apache Kafka is open source and maintained by the Apache Software Foundation under the Apache License, Version 2.0.
   
2. Hue (Hadoop User Experience): Hue is an open-source SQL Assistant for browsing, querying, and visualizing data with a web-based interface. It supports a variety of Hadoop ecosystem components including Apache Hive, Apache Impala, and Apache Solr, and is designed to make it easier for users to interact with data stored in Hadoop. It is also under the Apache License.
   
3. Hadoop YARN (Yet Another Resource Negotiator): YARN is the resource management layer of the Apache Hadoop ecosystem. It enables multiple data processing engines such as real-time streaming, data science, and batch processing to handle data stored in a single platform, ensuring that the growing number of data processing tools can coexist and share a single data set within Hadoop. YARN is an integral part of Hadoop, which is open source under the Apache License.
   
4. HBase: Apache HBase is an open-source, non-relational, distributed database modeled after Google's Bigtable and written in Java. It is developed as part of Apache Software Foundation's Apache Hadoop project and runs on top of HDFS (Hadoop Distributed File System), providing Bigtable-like capabilities for Hadoop. It is also licensed under the Apache License, Version 2.0.
   

These tools are widely used in the industry for big data and distributed processing applications, benefiting from strong community support and continuous development within the open-source model.