If you are logged in to Yahoo!, you can show your online presence on your website with an icon. If you use Yahoo! Messenger instant messenger and would like to stay connected with visitors on your website, Yahoo Online Presence is a cool way to display to them when you are online and available to chat / IM with them.
Just enter your Yahoo ID in thier form and they will generate a html code that you can copy and paste into your web document html code. Then whenever you are online and your Yahoo Instant Messenger is connected and available for chat, your website will display “I am Online” logo. If a Yahoo! Messenger user clicks your online presence, an instant message window will open for them so they can type you an instant message / IM. If you are not online, A message can be sent to you when you are online later.
Remember that this will reveal your Yahoo ID to the visitors on your site. If you wish to protect your privacy and original Yahoo identity, create a new Yahoo ID and use it for this purpose.