Conversion specialist Bryan Eisenberg just posted a great list of 69 Free or Low Cost Tools to Improve Your Web Site :
Here are a few of my other favorite tool lists:
SEO Guru Todd Malicoat’s summary of “Tool Time With Todd” twitter sessions:
An older, but still functional list of SEO tools: http://www.seocompany.ca/tool/seo-tools.html
Of course, the seoMOZ tool sets: https://moz.com/free-seo-tools
And then some excellent research tools from AdCenter Labs: https://advertise.bingads.microsoft.com/en-us/solutions/tools My favorite is the Commercial Intent Tool.