New? gmail tool

i don't know if i was unobservant or if this is a new addition to gmail (probably the former). you can send email from other accounts. so for instance i have a "professional" email that i put on my resume, my normal user name, and one i use for my bills and such. i have had the professional one and bills one forwarded to my normal one for a while now. but this week i discovered that i can be logged in to my normal one and reply/compose using the email address of the other accounts. and it can be set to respond using the account it was sent from so i no longer have to log out and log in as another user to send a reply. just what i have always wanted.