Stack trace with function calls and line numbers.

Source map support is used to reference original source code in cases where the code is minified.

Rollbar allows ignoring specific exceptions simply by adding the error message to the ignoredMessages array.

Rollbar can be integrated with Slack to send notifications of various event to a set Slack channel based on customizable rules.

Rollbar allows sending any log messages to Rollbar not just exceptions enabling you to go back and check for missed exceptions.

This feature is quite an interesting technical feat, considering that it shouldn't really be possible to get error information form 3rd party scripts. See this and this for discussions.

At least as of 2015-July-14. Many other prominent loggers are blocked, e.g. Loggly and LogEntries.

Free plan allows for up to 5000 events/month, customizable rate limits and 30 day retention.