A perfect example of a self spreading virus are those that travel over instant messaging clients such as msn. They automatically send themselves to anybody an infected user has on their friends lists and can be incredibly difficult to protect against at times.
My apologies if I'm wrong on this, but haven't most of those required the person to accept an incoming transfer and run the infected binary? I seem to remember one being self-executing, but I thought most of them just used IM networks as a means of transmission rather than infection.