Update: Sorry, doesn't work any more.
Yahoo! Status Checker is a mini web tool used to check status (online, offline) of a yahoo user. You need to enter Yahoo! ID or Email which you wish to check, into the textbox and press Go!. It'll shows status and also avatar of that user. To save the avatar right click on avatar image and choose "Save Image As" in Firefox. Under the avatar, there are few links like View Profile, Send a Message etc. which are used to interact with user or to know more information about the user.
If it shows that user is online, but in your messenger list it is still offline, that means user has set to appear invisible to you through stealth settings. Currently, I am working on it, to add more features to it. Hope you guys find it useful.