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The cheapest and perhaps most effective way may be to invest in some pizza and beer and have some friends over to your house that have never seen your website. Then let them just look around the site and offer comments while you take notes. Keep track of what they like/don’t like, pages that don’t get visited, how easily they navigate the site, etc. The more people you can get to do it, the more insights you will gain, and you probably won’t even spend $100 for it.