I'm not quite sure when I encountered the Tam Lin ballad.
One of my favorite modern retellings of this story is Pamela Dean's Tam Lin. Not only is it a great retelling that is true to the original work, it's also captures the university world very well. It also does a great job of portraying smart people who are unafraid of being smart. The book has so many literary and historical references that there are sites dedicated to annotating the book, such as The Annotated Tam Lin.
Other Tam Lin retellings I like:
The Perilous Gard by Elizabeth Marie Pope, set in 16th century England; and
Fire and Hemlock by Diana Wynne Jones, set in modern times.
If you're totally unfamiliar with Tam Lin, the best place to start would be a book that is more traditional, such as An Earthly Knight by Janet McNaughton or a picture book, like Jane Yolen's Tam Lin.
Please share your favorites! I would especially love to read one that totally turns the legend around, making Tam Lin and Jean the bad guys to the Queen of Faery's good guy. (Tho if it isn't out there, I may have to write it myself.)
For a movie related link: Roddy McDowall (one of my favorite actors ever) directed a movie version (AKA The Devil's Widow AKA The Devil's Woman) of this that I have never seen. It's the only film McDowall directed, and Hollywood lore says that McDowall did a great job but the studio treated him and his work poorly. It's not available on DVD, but was released on VHS.
The, queen of the fairies,
bade me with her to dwell,
And still once in the seven years
We pay a teind to hell.