[The Might]

1. Barliman, Butterbur, who forgot about the message he was supposed to send and so indirectly helped the Nazgul track Frodo
2. Goatleaf, Harry and
3. Ferny, Bill who were suspected by Aragorn of doing work for the Nazgul

I also though about Hobbits, but none really helped Sauron
I actually had a more or less strange idea
It might also be that the Horblowers indirectly helped Sauron. Because of his cultivation of pipe-weed, it spread in the Shire and eventually even Saruman decided to purchase some. The squint-eyed Southerner that was on his way to Isengard to bring the pipe-weed was caught by the Nazgul and revealed precious information about The Shire and Baggins.