Is anyone using anything like this (I'm really thinking of things that don't
cost tens of thousands of dollars, but I'd be interested in hearing about experiences with them nontheless)?
...select a hosted application and review the applications source, documentation and comments. Navigation between the three is quick and intuitive. When reviewing application source or documentation it is possible to post comments relevant to the specific sections. When reviewing source code, documentation or comments there is always a handy button for proposing new or additional comments. Behind this button there is enough intelligence to ensure that the new comment will be properly positioned and related to the source code or documentation that inspired the creation of the new comment.