Simon Helberg stars as Howard Wolowitz, the only one of the nerds that is not a doctor.  He is an engineer at Caltech and is friends with Leonard and Sheldon and frequently is over at their apartment.  Wolowitz mostly goes by his last name and still lives with his mother.

Most jokes with Wolowitz revolve around his failed attempts at womanizing, though he was given a girlfriend, Bernadette, in season three.

Howard Wolowitz Quotes

Sheldon: Is anyone else troubled by the Spider-Man theme song?
Leonard: Why would it trouble you? It's like your third favorite cartoon theme song.
Sheldon: It is, right behind [Sheldon sings "Inspector Gadget" and "Teenage Mutant Ninja Turtles" theme songs]
Sheldon: However, the Spider-Man lyrics posit that Spider-Man's Spider-Man does whatever a spider can.
Howard: Yeah, so?
Sheldon: I can think of many things that Spider-Man can't do that a spider can. One crawl in your ear and die, two legally leave Guatemala without a passport, and three have sex with a spider.

Howard: Why don't you put her in a kennel?
Raj: Why don't you put your mother in a home?
Howard: To be honest, she'd do better in the kennel.

TBBT Quotes

Sheldon: Why do you have the Chinese character for "soup" tattooed on your right buttock?
Penny: It's not "soup," it's "courage."
Sheldon: No it isn't. But I suppose it does take courage to demonstrate that kind of commitment to soup.
Penny: How'd you see it? You said you wouldn't look.
Sheldon: Sorry. As I told you, the hero always peeks.

It's very simple. Scissors cuts paper, paper covers rock, rock crushes lizard, lizard poisons Spock, Spock smashes scissors, scissors decapitates lizard, lizard eats paper, paper disproves Spock, Spock vaporizes rock, and, as it always has, rock crushes scissors

Sheldon