There's a good British film called the Hide that's similarily impressive on a small scale. Two actors, one location (a bird wtachers hide). Great little film.

That started life as a play as well.

Since we are naming out small cast/minimum location films, I'll go for the obvious "Dial M For Murder", which, like "12 Angry Men" was adapted (very succesfully) from a stage play. If I remember correctly, there are a couple of exterior shots but 90% of it takes place in one room.

It would provide great inspiration & ideas for anyone looking to write such a piece.