Depends on what you're looking for, but there's two basic options - Adaptavist's Community Forums and Atlassian's Questions addon.
They are different, forums really is aimed at replicating the multi-user, multi-thread structure most other forum software implements, and Questions is very much more like a support/Q&A site. I'd try looking at both though.
(Disclaimer here - I work for Adaptavist. Please don't take this as a sales pitch, I'm just pointing out that it's intended to do forums in Confluence and might suit you. I don't actually work with it much myself, I'm in a different area, although I have worked with older versions and it worked well then)
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