S4: S4 is a general-purpose, distributed, scalable, partially fault-tolerant, pluggable platform that allows programmers to easily develop applications for processing continuous unbounded streams of data.
Hedwig: Hedwig is a publish-subscribe system designed to carry large amounts of data across the internet in a guaranteed-delivery fashion from those who produce it (publishers) to those who are interested in it (subscribers).
Storm: Storm is a distributed, reliable, and fault-tolerant stream processing system. Its use cases are so broad that we consider it to be a fundamental new primitive for data processing. Introduction slide
Flume: Apache Flume is a distributed, reliable, and available service for efficiently collecting, aggregating, and moving large amounts of log data. Its main goal is to deliver data from applications to Apache Hadoop’s HDFS.
Scribe: Scribe is a server for aggregating streaming log data. It is designed to scale to a very large number of nodes and be robust to network and node failures.