Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
logstash - logstash is a tool for managing events and logs.
memcached - High-performance, distributed memory object caching system
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
rsyslog - Rsyslog is an enhanced syslogd supporting, among others, MySQL, PostgreSQL, failover log...