Idina Menzel had a very recurring role in two of the seasons: she was the director of the show choir at a rival high school (I forget the name of the HS, but the show choir was called "Vocal Adrenaline". In a ginormous plot twist she turns out to be Rachel Berry's BIRTH MOTHER (Rachel has two gay dads, one played by Jeff Goldblum) which nobody knew until worlds collided. She wants a baby, and ends up ADOPTING Quinn and Puck's baby, Beth (Puck likes KISS).
She was fantastic. We loved her on Glee. There's an episode where she and Rachel Berry sing the duet from Wicked,
For Good - I cry, it's so beautifully done. Holy cow, the pipes on those two ladies.
I am the world's oldest Gleek. No regrets.