<< Have you ever realized you mistakenly left someone important out of an email, or just spent too much time trying to decide who from your long list of contacts to include? Well, some of us on the Gmail team feel your pain, so we wrote a new Gmail Labs feature called "Suggest more recipients."

Once you've enabled it from the Labs tab under Settings, you'll see suggested recipients while composing messages. Gmail will suggest people you might want to include based on the groups of people you email most often. So if you always email your mom, dad, and sister together, and you start composing a message to your mom and dad, Gmail will suggest adding your sister. >>

http://gmailblog.blogspot.com/2009/04/new-in-labs-suggest-more-recipients.html

Useless but good promo for FF.

That is useful. But not as useful as more signature choices, and HTML signatures, areas sadly lacking in GMail.

Just use Canned Responses for HTML & multipule signatures...