logstash - logstash is a tool for managing events and logs.
rsyslog - Rsyslog is an enhanced syslogd supporting, among others, MySQL, PostgreSQL, failover log...
log.io - Real-time log monitoring in your browser
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Gravwell - Gravwell is an all-you-can-ingest data fusion analytics platform that enables complete context and root cause analytics for security and business data.