Also a great library for onboarding purposes is Hopscotch [1]. It is allows you to create a interactive tour through the UI.
We are using it to introduce new users when launching the webapp the first time (Because having a 100% intuitive UI is a hard thing). We also made a study which showed us that using a onboarding mechansim really helps the user to get started and does not scare them away (which was a concern of some people).