Who played Joan Holloway in 'Mad Men'?

In which TV series is Shawn Spencer a main character? In which TV series is Don Draper a main character? Who played Peggy Olson in 'Mad Men'? In which TV series is Peggy Olson a main character? 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?