How do you increase free traffic to a website?

I don't know what traffic generation strategies you are already using. However, blog commenting or even PPC are ways that could be employed to drive traffic. The first one is a free method while the second one is not.

Blog commenting plays an important role in improving traffic and Google page rank.But blog commenting on general blogs are penalized now a days.But you have to comment on relevant blogs means blogs based on your niche.Other than blog commenting you can use forum posting,director submission,press release,article submission,guest blogging etc.