This is a top list of popular Webmaster Forums curated by Vladimir Prelovac and sorted by Alexa rank and Topics/Category.
Unfortunately this list is from 2010 and a little bit outdated but it still helps to get an overview of relevant Webmaster Forums.
> www.prelovac.com/vladimir/best-webmaster-forums/
Vladimir Prelovac also curates another List of the best social media sites.