Right off the top of my head -- plays by Oscar Wilde (The Importance of Being Ernest), G.B. Shaw (Major Barbara), William Inge (The Playboy of the Western World), Restoration plays (heavy on the costuming, though, and not easily parlayed into modern garb), Greek classics, Gay's The Three Penny Opera, Ibsen, Checkov, and many others I can't think of at the moment. For non-English plays, however, you will have to pay rights for the translations unless you find old translations in public domain. Try the Gutenberg project, or do a search for "public domain". I hope this helps.