Introduced in March-April 2010, Habbo ID uses email addresses as the new system to log in. This was enforced on August 26th, no longer allowing usernames to be logged in. (Instead, you need to use your e-mail). After players verify their email address, they can use it as their ID.
Players could also add another accounts to their email address, via the settings page. (Before, you would get a menu immediately after you logged in, of all of your Habbos)
In the hotel, you can see your own e-mail above your user name, when you click on yourself. However, no one else can see your e-mail as you can.
Habbo ID is heavily criticized by many players, claiming there are many bugs and flaws in the system and that it takes much longer to log in using their email address instead of previously their username.