If you want to have a forum but don’t want to spend any money you can check these services out.
Note that i haven’t tested them personally but they seem to be pretty good.
I have also listed some forums that use them so you can see how they look in action.
(and check out how many ads are displayed etc.)

Also consider to check out The Big List Of Forum Software

ProBoards

Forums That Use Proboards

• Cactus
• Dan-Ball
• HG101

Further Info

• Features
• Support

ZetaBoards

(previously InvisionFree)

Forums That Use Zetaboards

• Mushroom Kingdom Fusion
• Directory

Further Info

• Features
• Themes
• Documentation
• Support

FreeForums.org

Forums That Use FreeForums.org

• GunGirl2

Further Info

• Features
• Support

reddit

reddit allows you also to create your own community.

• reddit.com/r/IndieGaming/
• reddit.com/r/gamedev/
• reddit.com/r/freegames/
• reddit.com/r/opensourcegames/
• reddit.com/r/WebGames/
• reddit.com/r/linux_gaming/

• reddit.com/r/pcgaming/
• reddit.com/r/gaming/
• reddit.com/r/gamingnews/
• reddit.com/r/playitforward/
• reddit.com/r/gamereviews/
• reddit.com/r/gamenostalgia/

• reddit.com/r/gamedeals/
• reddit.com/r/steamdeals/

Further Info

• FAQ
• Help
• Wikipedia Entry

Further Reading

• Forum Software Reviews
• Forum Matrix