1) Columbia Teachers College offers a unique "INSTEP" program in which you can get an MA in Teaching English, Social Studies, or Music. You commit to 3-6 weeks for three consecutive summers in NYC (not too shabby!), and you also have to complete minimal online coursework throughout the year. This program is the creme de la creme, but is also the most expensive. They do offer some scholarships to new students. (This is the one I am starting this summer! Hurrah!)
2) Colorado College offers an "MAT for Experienced Teachers." This one seems like a hidden gem! You can get your master's over three summers, and there are also several during-the-year requirements. The program is small, but from my limited contact with them, it feels personal and flexible. They offer a 50% discount to working teachers, which makes the program surprisingly affordable.
3) San Diego State University offers an MAT in Language Arts also over three summers. Students take classes in literacy, children's literature, and research. There are no requirements during the school year, as far as I can tell. And as you could probably guess, it's incredibly affordable.
4) UC Irvine has a pretty rigorous-sounding Masters in English. For two summers, teachers attend classes for eight weeks straight (which, let's be honest, is basically the whole summer). However, there is absolutely no coursework during the school year, and the third summer is spent working on a thesis. This program appears to be the most bang for your buck. You complete the whole program for under $7000 total, and you get a degree from a nationally respected English department.
5) Middlebury College - Bread Loaf School of English offers one of the most well-known, and largest, summer Masters in English. Students get the option of taking courses in Vermont, New Mexico, or Oxford (woo woo!), and there are courses in creative writing and theater arts as well. The catch here is that it takes FIVE summers, and that can add up. But everyone I know who has done this program has adored it.
6) Sewanee School of Letters offers a program similar to Bread Loaf's-- in that you can get a Masters in English in five summers-- except there is no campus flexibility. However, the University of the South campus is supposed to be beautiful, up in the mountains of Tennessee, and the MA students work alongside the MFA students each summer.