I am a Canadian who has spent more than 6 months in the US on TN states. When I visit, can I enter the United States visa free?

ESTA in my understanding does not apply to Canadians. The question to a CBP officer is whether you would be a bona fide visitor. Usually, a bona fide visitor spends less than six months per 365 days in the US. Having been in TN status for six months (presumably within the past 365 days) could raise concerns on the purpose of your stay. If you have a legitimate reason for coming back so soon as a visitor, you can attempt entry visa free since six months is not a hard and fast rule. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.