It seems to me that many people, particularly authors, find their way to Carterhaugh despite the narrator's warning ("I forbid you, maidens all" and all).
For my part, I discovered Tam Lin through Pamela Dean's novel,and after some time of subsequently feeling like only she and I were really aware of the story, realized that there were really quite a lot of versions, and that quite a lot of people are at least a little bit obsessed with the ballad. So I want to record here a few of those retellings, in the order I encountered them, to the best of my ability. Doubtless there are many more, and maybe you'd like to tell me about them in the comments? I know I would like it if you told me about them in the comments.
Pamela Dean - Tam Lin
Set on a college campus with an eerie Classics department and students who recite Shakespeare.
Janet M. Naughton - An Earthly Knight
In an attempt to histori-cize the ballad, she sets it in 12th century Scotland. Plus, there's another ballad thrown in for fun.
Elizabeth Marie Pope - The Perilous Gard
Actually mentions the ballad, while the heroine (called Kate here) attempts to determine the nature of Faerie. I remember reading this book, and thinking I should just stop trying to write faeries, because Pope had done it so very well first. Honestly one of the most interesting interpretations ever.
Diana Wynne Jones - Fire and Hemlock
Involves a lot of tracking down memories, and somewhere in them, Tam Lin and Thomas the Rhymer get to play.
Patricia A. McKillip - Winter Rose
Set in a magical medieval-ish forest, with gorgeous writing.
They're both pretty much straight interpretations of the ballad, but without, you know, the whole pregnant part.
Jane Yolen - Tam Lin
Susan Cooper - Tam Lin
Holly Black - Tithe
It's not Tam Lin, per say, but she does explore the tiend, and the political implications of messing with it.