Or Option-Command-B from the keyboard, to toggle the Bcc (Blind Carbon Copy) field on and off.
Thank you for knowing enough to look for the Bcc field, seans9. I wish more of my friends would think of this. When somebody I know sends e-mail to everyone they know, I have no business knowing the names and e-mail addresses of all of their other friends, and their other friends have no business knowing mine.
By the way, you don't have to fill in the To field at all when you use Bcc, so don't feel obliged to fill in anybody at all as the visible recipient. Some people, not knowing this, send to themselves with a Bcc to their other recipients, but this is not necessary.