Pidgin a.k.a Gaim from http://pidgin.im/ is a great instant messaging client and is available

Pidgin a.k.a Gaim from http://pidgin.im/ is a great instant messaging client and is available for all platforms including ubuntu linux and supports Yahoo, MSN, Google Talk, ICQ, any jabber based instant messaging service and more instant messaging services

Open a terminal and issue the following command

sudo apt-get install pidgin

Add your yahoo messenger account to pidgin and enjoy chatting