Forum Software

I am on the look for a new forum software. Mainly want something simple but allows user levels and it would be a plus if it worked well with WordPress right out of the box so there would not be a need for multiple logins. I know there is a lot of bridging software available out there for certain forum software so that part prob wont be a problem. I have used vBulletin, Invision in the past which are very full featured… but they cost money. Anyone have any ideas?