BuddyCheck will show you the real online status of your Yahoo! Messenger friends. It will regularly check to see if they are online, even if they are invisible or are using Stealth Mode. It will notify you using a tray area balloon when a contact signs on or off.
It does this by sending special messages to the Yahoo! Messenger server from a separate account which you must create. You could use your regular Yahoo! Messenger account for this, but this is not recommended since only one application can be logged in at any given time into one account.
You can customize the period after which each contact is checked again for change, and you can set individual checking periods for some of your contacts. You can also easily import contacts from any Yahoo! Messenger account that you have.