People, it seems Gmail labs is rolling out features like anything. Just after releasing support for inserting inline images, Google labs has rolled out another feature that will auto suggest recipients while composing an email. Based on your email composing history, it will figure out the probable recipients for your email.
In order to use this features, you have to enable the same from settings->Labs->Suggest more recipients
Nothing special about it but it is truly innovative. You can find Gmail announcement about this at this address.