Support forums exist for seemingly every form of popular technology and even for most forms of less popular and mainstream technology. They can be dedicated to specific trades or areas of knowledge, to specific brands and products and more.
From web hosting to consumer electronics, from official communities to unofficial ones, support forums often represent the greatest collection of concentrated knowledge aimed at whatever it is that they cover.
I’ve participated, at various levels, in a number of them over the years and have run phpBBHacks.com for 10 and a half years and PhotoshopForums.com for 8 and a half. My experience has led me to believe that truly great support forums often share 3 specific principles.