Tough thing to pick a few good teaser scenes to introduce folks into Peep Show, but this is one of my favorites:http://www.youtube.com/watch?v=OZrTKdntMUo
Mark is trying to befriend Jeff, one of Mark's colleagues who's dating the woman Mark is obsessed with at work, Sophie... so that Mark will eventually try to steal Sophie for his own. Mark's best friend and roommate, Jeremy, finds Jeff to be his ideal kind of friend, unlike misanthropic, socially awkward Mark.
The poker scene best illustrates Mark and Jeremy's personalities. Having a passive-aggressive inferiority complex, Mark folds despite having a fantastic hand. Meanwhile, Jez hasn't a clue what he's doing and doesn't mind taking a chance.
This scene also has one of my favorite inspirational quotes: "...and when the going gets tough, you need to roll out the big potatoes... let's eat."