Although my personal tastes run toward open-source, multiprotocol support when choosing a messaging app, there's a lot to be said for Yahoo Messenger 9. For one thing, if most or all of your friends are on YIM, then there's no reason to forgo the benefits of using the main client.
YIM 9 retains all the voice, chat, and drag-and-drop capabilities of its predecessors, and adds a lot more visual pizazz and some key functionality. One is contact importing, which scours your other IM, e-mail, and social-networking accounts for friends who are also signed up on Yahoo IM. You can then ask some or all of your friends to add you and can invite others to sign up. Another favorite new trick is the ability to see images and videos displayed inside the chat window when you or a pal drops in a public URL.
Other notable changes include the visual skin picker to switch up the look, and Pingbox, a chat window site owners can embed to chat with visitors in real time. Users should note that the download stub will finish installing the program online, and that the download bundles the Yahoo Toolbar. There are a lot of extras rolled into YIM, and you can opt out of them by choosing Custom Install after running the executable. Be wary of the heavy-handed Yahoo branding and keep an eye out for otherwise hidden smilies, but these are minor quibbles.