Recs.
Updated
Supports local variables in stack traces, source maps. Also supports logging from many other languages. Free plan for <5000 events/month.
Specs
Pros
Pro Integrates with Slack
Rollbar can be integrated with Slack to send notifications of various event to a set Slack channel based on customizable rules.
Pro Not blocked by ad blockers
At least as of 2015-July-14. Many other prominent loggers are blocked, e.g. Loggly and LogEntries.
Pro Multiple integration options
Asana - Create and resolve Asana tasks
Bitbucket Issues - Create and manage Bitbucket Issues
Campfire - Get alerts in your Campfire room
Flowdock - Get messages in your flow
GitHub Issues - Create and manage GitHub Issues
HipChat - Get alerts in your HipChat room
JIRA - Create, resolve, and reopen JIRA issues
PagerDuty - Trigger and resolve incidents in PagerDuty
Pivotal Tracker - Create and manage Pivotal Tracker stories
Slack - get alerts in a chatroom or group
Sprintly - Create and manage Sprintly items
Trello - Create and manage cards in Trello
VictorOps - Trigger and recover alerts in VictorOps
Webhook - Post JSON payloads to arbitrary URLs
Email - get notifications and daily summaries
Pro Source maps support
Source map support is used to reference original source code in cases where the code is minified.
Pro Error filtering
Rollbar allows ignoring specific exceptions simply by adding the error message to the ignoredMessages array.
Cons
Con Old-school UI
Built with the default Bootstrap theme. Looks like 2011.