My wife and I just finished our NFP classes and I believe fertility friend is still the populart free one that is relatively easy to use. though she thinks there are others.
I’m not sure if I which sites I would be comfortable using, I saw www.mymonthlycycles.com linked from contracept.com but I doubt the pro-life catholicity of the site because contracept is obviously not.
I’ve spent a few minutes searching and it took me a while to stumble across fertility friend so I wish you luck and God Bless,
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