Topic of the Day: if you're talking about off-Broadway plays and musicals that we've actually
seen off-Broadway, then I haven't. Unless you include STOMP, which I did see in Greenwich Village in 1995, but it's not really a "play" or a "musical." If you're talking about
cast recordings of off-Broadway productions, then I'll say:
Dames At Sea
The Fantasticks
Forbidden Broadway (in all its iterations)
Howard Crabtree's When Pigs Fly
Howard Crabtree's Whoop-De-Doo
Little Shop of Horrors
Lucky Stiff
No Way to Treat a Lady
Nunsense
Oil City Symphony
The Threepenny Opera
Tuscaloosa's Calling Me, But I'm Not Going
and mentioned by Elmore:
Ruthless (L.A. is off-Broadway, right?

)
And many others.