We use KnowledgeBase Manager Pro. It is really good knowledge base software that we have used and really happy with it. Tried to use several other solutions, but in most cases they are unclear and buggy
I've used many over the years - kayako, whmcs, modernbill, cerbrus, intelidesk and more. I prefer using a KB system that is integrated with a helpdesk so that the knowledgebase will actually produce potential answers to customers before entering a ticket.
We use WHMCS here, but Kayako's system was pretty great in years past.