Clientside JS error reporting services

I don't think enough developers use these currently... and are you sure your users never hit corner-case exceptions in your app?
It's certainly possible to roll your own with TraceKit and Google Analytics, these services dramatically improve the reporting side for you.

The List  

Other: - open source - most comprehensive stacktrace library

Update: Edited this list maybe 7 times since first published. Last updated March 2014
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