I second both "Schindler's List's" with the breakdown scene, that is one of the most powerful scenes in any film I've seen, and "A Few Good Men's" "you can't handle the truth" scene. Cruise vs Nicholson with both of them on top of their game, probably the most famous courtroom scene in movie history.

Others? Memento

Star Trek II - Leonard Nimoy and Willam Shatner as he finds Spock dying.

12 Angry Men - Convincing the 12th Juror.

Apollo 13 - Tom Hanks when he flips out on VOX (not realizing his radio is still on) and rips off his medical things.

Good Will Hunting - Robin Williams' speech to Will.

Hotel Rwanda - Don Cheadle's character telling the truck to go without him

The Pianist - The German Colonel finding him and having him play

Saving Private Ryan - Tom Hanks explaining why he's doing the mission to save one man.

To Kill A Mockingbird - Pretty much all of it.

Gregory Peck enforcing his pessimistic bomb expert in Guns of Navarone

Good call.

Pacino in The Merchant of Venice during his "Hath a Jew not hands..." speech.

21 Grams: Naomi Watt's breakdown in the kitchen with Sean Penn.
In the Line of Fire: Clint Eastwood remembering the time when JFK was shot
Infernal Affairs: Tony Leung's reaction after seeing "someone" thrown off a building

Pacino seems to be on this list quite a bit.....

I thought Tom Hank's bits on the island in Castaway was very good. Tom Hanks went for considerable time without saying anything, but the acting was so good that you just didn't seem to need it.

(It effectively had a "monologue" that went on for around 80 minutes, which requires considerable ability to pull off convincingly).

this one is kind of obscure, the "stabbing" scene in Streamers (Altman film from 1983) its very real and the reactions from everyone to it and the aftermath is very well done. Not sure why i thought of that one but its what came to mind

PLUS:

Naomi Watts for a number of films, mainly the aforementioned 21 Grams scene...she might be our best actress of the last ten years