Sometimes when you need to fire off a quick e-mail reply to someone you hit the Reply button… quickly. As you compose your brilliant reply you realize that what you really wanted to do was “Reply To All”. Instead of forcing you to close your composition window and start over, Mail.app features a handy “Reply To All” button right in your message composition window toolbar. Simply click to add the recipients and carry on with your brilliance.
I simply cannot believe I have been using Mail.app for the better part of a decade and never knew about this. Even if it is a new feature in Snow Leopard, shame on me for not seeing this for the past couple years.
I will be using this for nearly every "reply all" situation, as I've always ended up sending a message off to just the sender before today.
Patrick Rhone adds:
Also, it’s a toggle button. If you hit reply-all, then the button will switch it to reply and vice-versa. Also, the keyboard command equivalents work here as well.
[hat tip: Minimal Mac]