Each project can include the ability to leave suggestions or to notify about bugs. Doing this may not even registered users. There are several options. If a web project - then you can directly on your site to place the code generated for this project, and in its place on the site will be a feedback form with the chosen design. All messages from it will automatically fall into the section "Proposals" for this project.
Based on the data entered once can be a problem or send a correction of an error. When new messages arrive, the managers of these projects will receive the notification for operational control.
Or you can go to the site of the section "Bugs & Suggestions" and leave there for his wish. In this case, you can even join to post screenshots or other files.