Facilitating web page error reporting

Are there currently any products for facilitating reporting errors in web pages?

Problem description

A end user is browsing through your site, and stumbles upon a page which for him has an error or bug. It might be a mother-of-all-stacktraces shining at him, a number which is wrong or single space missing. Often, the end user is the only one who can see the problem, and it is difficult and timeconsuming to report through email or on the phone.
Preferrably, the user can send a screen shot. But it is a difficult process to explain, several manual steps involved, and the state of the application cannot be captured by a screen shot.

Possible solution

A "report bug" link which can easily be placed on every page, that when pressed, creates a report containing
* The offending web page as html
* A dump of all relevant state for the page
* A field where the user can type in what's wrong
* A crude marking device to easily tell where the error is located
The report can then be emailed and fed to an issue reporting system.

Relevant technologies

A javascript library or reusable SiteMesh decorator or similar. Preferrably as little intrusive as possible.

Does this already exist?

Please tell me if it does! :)