There are others, but those come to mind right away. I wouldn't call some of the songwriters who haven't "made it big" yet amateurs, though. They're writing on a professional level, yet "professional"producers haven't given them a chance yet.
Me. My friend and I wrote a musical called "Alice". It's a new take on Alice in Wonderland. We produced it at a local park in our town. The 29 people who bought tickets seemed to enjoy it.
Theatre is my life. No one can take that away from me.
For your consideration, Thomas Ades, whose opera version of The Tempest is garnering amazing reviews. (Going to see it later in its run and I'll report back-what I've sampled I loved.)
I still can't believe that Jimmy Webb hasn't had a show on Broadway yet. I love his songwriting, anyway, but as he makes clear in his book on composing, he seems to grasp the differences between pop writing and theatre writing.
He's written a number of musicals, and a few songs have been released, like on Michael Feinstein's excellent Webb album. There have been regional productions of his adaptation of Ray Bradbury's Dandelion Wine--which I'd love to hear in full. And of course he was involved in two Michael Bennett productions that never made it--The Children's Crusade, and Scandal (which I'd *love* to hear).
Since this thread has been bumped, I see that I missed Jay asking me what I thought of Previn's adaptation of Streetcar Named Desire. Unfortunately, it doesn't really work for me--at least on DVD (and yet I liked it enough, I guess, to buy the DVD). The music is interesting, but I just don't think the piece gains anything by becoming an opera (and Williams' had enough music in his language, that setting, often comparitvely banal, lyrics to opera robs a lot of the work's poetry). I also find Fleming unintentionally hilarious in a few parts (in the book When Blanche Met Brando, the author is VERY hard on the work--he hates it--but he does give some funny quotes from Fleming which suggest she didn't get the character at all).
I don't know if Williams would ever work musicalized (as a musical or an opera). I know a couple of William Inge plays have had somewhat successful opera adaptations (though the musical of Picnic was a big flop), and they share similar qualities, so I could be wrong. I would be curious to hear the opera version of Summer and Smoke that Lanford Wilson did the libretto for (I can't even remember who composed it).
My favorites are probably Jeff Blumenkrantz and Kerrigan-Lowdermilk. I also like Carner and Gregor's work that I've heard, as well as Lance Horne.
I've been really loving Kerrigan-Lowdermilk in particular recently. I think they write really well as far as communicating what it's like and how it feels to be young without writing in a way that only appeals to people who are that age, if that makes any sense. I do like some of Pasek and Paul's work but I also think that sometimes it's the sort of thing where you can tell they were writing for college-age people and it can be a little lacking in maturity (and I say this as someone who is 21 and in college).
I'd love to make a case for some awesome Canadian writers whose work might be lesser known internationally. While several Canadian shows have made it to the Great White Way (Anne of Green Gables, Drowsy Chaperone, Story of My Life), the majority remains our little secret.
I recomend looking into the work of: Paul Sportelli and Jay Turvey (Little Mercy's First Murder, Tristan, Maria Severa): http://www.jaypaulproject.com