Currently we use Trac. Trac is an open source, web-based project management and bug-tracking tool, inspired by CVSTrac. It is developed and maintained by Edgewall Software.
There is also old GRASS Request Tracker.
The GForge and RT GRASS trackers can be used to examine the previously resolved issues, and those that were still in progress as the new system was released. It would be good to move all the tickets from the former trackers to the new one, but nobody has volunteered yet. If you are the one, please contact the admin to arrange it.