An incomplete list of applications and organizations using librdkafka to interact with Apache Kafka:

• kafkacat - Apache Kafka swiss army knife
• Wikimedia's varnishkafka - Varnish cache web log producer
• Wikimedia's kafkatee - Kafka multi consumer with filtering and fanout
• NVIDIA DeepStream
• rsyslog - output and input modules
• syslog-ng
• collectd
• kafkabridge - The kafka client sdk, includes C/C++, PHP, Python, Golong, etc
• logkafka - Collect logs and send to Kafka
• rafka - Rafka is a gateway service that exposes Kafka using simple Redis semantics.
• redBorder
• Headweb
• Produban's log2kafka - Web log producer
• fuse_kafka - FUSE file system layer
• node-kafkacat
• OVH - AntiDDOS
• otto.de's trackdrd - Varnish log reader
• Microwish has a range of Kafka utilites for log aggregation, HDFS integration, etc.
• LATOKEN - crypto currency exchange on Kafka
• aidp - kafka consumer embedded Lua scripting language in data process framework
• Yandex ClickHouse
• NXLog - Enterprise logging system, Kafka input/output plugin.
• ITRS Geneos - Real-time application performance management, publish data to kafka.
• MagicLab is using librdkafka in the biggest dating projects over the world such as Badoo, Bumble, Lumen and Chappy.
• OmniSciDB - Open-source, GPU-accelerated Relational Database
• Tarantool Kafka - Connector library for Tarantool In-Memory Database
• Ceph - Distributed storage system using librdkafka to publish bucket notifications to Kafka, from its Object Storage Gateway
• large unnamed financial institutions
• and many more..

If you're using librdkafka and want to share your success, feel free to edit this wiki page and add your project.