Gaim : Gaim is a powerful Yahoo Messenger alternative, being recently renamed to Pidgin after some federal problems with more famous companies. However, Gaim is still available for download in version 2.0 beta 6 but you're also required to install GTK+ for Windows as an add-on for the stand-alone program. Gaim offers support for most of Yahoo Messenger functions, including file transfer and emoticons but it also includes some new tools that can be very useful for the chatting experience. For example, the Psychic plugin helps the users find out when another client of Yahoo Messenger starts to type a message even before he presses the enter button.
Trillian : Trillian is probably one of the best Yahoo replacements, helping users to connect to multiple networks including the giant’s portal. The program allows you to send files to your Yahoo contacts but it provides a different set of emoticons and sounds. The interface is 100 percent changed because Trillian enables you to customize the appearance by using different styles or themes. At this time, the developers are working on a new version of the application that will include support for many of the Yahoo Messenger editions and made them fully compatible with your client.
I think these two programs are the most powerful in their category so, if you want to use an instant messaging client other than Yahoo Messenger download them at the links provided (Click on the orange names above).
However, if you still want to chat even easier, without having to install any application, you can use one of the web-based clients that help you communicate on the Internet from an online Interface. Although Yahoo's Web Messenger is regarded as an easy way to do that, you can also use Meebo, a solution that supports a lot of messaging protocols.