Official site

• http://www.graylog2.org 

Comments:

• Open Source free powerful analytics tool.
•  Graylog2 enables you to unleash the power that lays inside your logs. Use it to run analytics, alerting, monitoring and powerful searches over your whole log base. Need to debug a failing request? Just run a quick filter search to find it and see what errors it produced. Want to see all messages a certain API consumer is consuming in real time? Create streams for every consumer and have them always only one click away.
• Used by both startups and major corporations all over the world. 

Start-ups using: 

• 'txtr 
• taulia
• UserVoice
• TicketFly
• Kiva 
• entp
• Backupify
• and many more...