Yahoo Online Checker - is a free Desktop tool that allows you to mass-check the online availability of Yahoo Messenger users without having to login or to have them in your friends list.
You can check any number of Yahoo ID's from a simple text file, and let the program check if they are online or not.
Additional you can save the Yahoo Avatar (regardless if the user is on or offline) and send them personal messages from the Program without having to open Yahoo Messenger.
You can check up to thousands of Yahoo ids - there is no real limit. In order to send personal messages to the yahoo id's you must set up the login data under the "Login Button"
Windows Server 2008