I think it depends more on your web server whether you installed Mod rewrite. Most free forums support Mod rewrite.
If you're referring to the most SE friendly forum platform, I could say phpBB is very SE friendly if you integrate some SE modifications, however I heard myBB is also a good SE friendly software, which is completely free. Meti
For those of you who said vbulletin - vbulletin is not free. The post created asked for the best seo forum software from a free provider.
That is because they have excelled in their promotion and now have a steady growth, for new starters, it would be good.