Who played Howard Wolowitz in 'The Big Bang Theory'?

In which TV series is Joan Holloway a main character? Who played Leonard Hofstadter in 'The Big Bang Theory'? Who played Penny in 'The Big Bang Theory'? In which TV series is Penny a main character? Who played Raj Koothrappali in 'The Big Bang Theory'? Who played Blair Waldorf in 'Gossip Girl'? In which TV series is Blair Waldorf a main character? Who played Serena van der Woodsen in 'Gossip Girl'?