I think alert() has been gone for a while. firebug/firebug lite and console.* seems like it's on the verge of a de-facto standard.
I agree, firebug killed alert() a while back
A useful addition to this would be the ability to automatically POST the results to some URL so you can be notified when client-side errors occur, probably because you broke something.
i'm sure one could make that happen on their own. if it were to come with this, it would make it no longer a pure js deal