The Museum of HP Calculators has a forum with areas for general HP discussions, the HP Prime and Classified Adverts, as well as software libraries for the HP-41C, HP-65/67/97 and HP Prime.
If you want to a forum specifically about the HP-41 hp41.org has one here.
Swiss Micros also have a forum dedicated to their machines here.
The HPCC Committee decided to remove our little used 'new' forum on 12th November 2022. If you want to look at our 'old' forum, go to the Wayback Machine.