There's this really good one, I don't know if you've heard of it. It's called Warrior Forum.
Do a search on Google, I'm sure you'll be able to find it.

Good luck!

sorry i didn't get your question? what do you mean by free forums? are you looking for marketing related forum where you can get information about Adsense?

I think you are looking for free forums software. If yes, you can use phpBB, MyBB, SMF which are free and powerful. there are many other free forums platforms too that you can use for free.