not likely to be any free services for SEO.
if your site is your phpbb board then you don't need any special services for SEO. there is nothing wrong with the SEO of phpbb out of the box.
the only changes you might want to make are to make sure that your forum titles and topic titles have relevant keywords in them.
then, you need content, content, content that is relevant to whatever your subject matter is.
there is no secret to getting better rankings in google. content relevant to whatever your subject matter is. if you have that then you automatically have relevant keywords in that content.