I'm sure this thread has been done before, but I'm wondering what suggestions people have. My favorites (focusing on older/more obscure ones for now, although you're welcome to add anything):
The Caine Mutiny: This is only a law movie for the last third or so, but Bogart breaking down on the witness stand is unforgettable. I think this movie is better and more believable than the similar A Few Good Men, which is another must-watch movie about law and the military.
12 Angry Men: The greatest movie about jury duty not starring Pauly Shore.
A Dry White Season: Marlon Brando's last great role, as a British barrister fighting apartheid in South Africa.
Judgment at Nuremberg: This movie is so tied to the courtroom that it's amazing how moving it manages to be. Maybe the best Holocaust film ever made, as well as one of the best legal movies.
To Kill a Mockingbird: This has to be the top. I get goose bumps every time I see Atticus Finch walking out of that courtroom.