Eduardo's scene at the Facebook offices - from smashing the computer to almost punching Sean, who flinches in cowardice even when not actually hit. Eduardo is a complete Badass.
Eduardo: I like standing next to you, Sean. It makes me look so tough.
Mark's entire "You have part of my attention, you have the minimal amount" rant. Awesome enough to be a little scary. Even the other actors acknowledge on the DVD Commentary that it's pretty chilling.
Rashida Jones' character ripping apart Zuckerberg's facade with one sentence: "I don't think you're an asshole, Mark. You're just trying really hard to be".
The scenes where Zuckerberg gets the Eureka moment for ideas that would eventually become cornerstones of Facebook's success are really cool, especially the one where he chances upon the idea to add a relationship status to the profiles, when a classmate is asking him to help him ask a girl if she is single.
While Daddy Winklevoss has only a handful of lines, his one big one is pretty indicative that, regardless of how you feel about his sons, he's a pretty awesome dad.
"No, don't apologize to me after losing a race like that. Don't ever apologize to ANYONE after losing a race like THAT."
The scene in which Mark creates Facemash, and eventually crashes the entire Harvard network through sheer volume of traffic. The scene itself should also be considered a crowning moment for the filmmakers (and Trent Reznor) for making a scene featuring nothing but people looking at computer screens into one of the most tense, exciting and well-crafted scenes in any film.
The entire verbal beatdown of the Winklevoss twins by Larry Summers.