Paul Elliott and I created and actively maintain Informant as a usability analytics tool.
We built Informant because we saw a need going unfulfilled: users make mistakes, triggering validation errors, and traditional error metrics don't typically track those errors. Even a well-maintained, error-free application may still be driving users away through frustrating interface design.
Informant exposes information about which validation errors users trigger, and how they trigger them. If users are forgetting a field, that's one type of problem; if users are consistently typing the wrong thing, that's another. And if Informant is watching an app that acts as an API server, it's also invaluable in catching API validation errors.
Informant is available for independent installation or as a Heroku addon, and is compatible with any app that uses ActiveRecord.