Sentry is a log aggregation tool that really shines when paired with applications that have the mentality that any log message at or above warning requires human attention. It can be thought of as a replacement to automated emails from syslog.
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Pro Completely open source, with a hosted version available
Sentry is a completely open source tool. It's free to use if you want to run it on your own. If you want to avoid the overhead of running and maintaining another server, it also has a hosted version available at www.getsentry.com
Pro Source maps support
Has support for source maps.
Pro Consolidates similar errors and makes resolution easy
Pro Email/Slack/GitHub/Hipchat integration
Pro Can create projects and teams
A single Sentry server can be used for various teams/products.
Pro Has an official Docker image
Con Not really a logging solution
Sentry collects exceptions, it's not a solution for log aggregation.
Con Can't filter by browser versions
Sentry only filters by browser names and not by browser versions (for example: Firefox, instead of Firefox 27, Firefox 28 etc...)
Con No free cloud hosting
Unfortunately there is no free plan for cloud hosted solution. Only self-hosted solutions are free to use.