Yahoo! Messenger is yet another InstantMessenger system. In addition to chat, it will provide notification when your Yahoo Mail account receives e-mail.
See http://messenger.yahoo.com/
It has some advantages over ICQ, like: contact lists always on the server (you can have contact list in servers in ICQ for some time now ) and you are able to change the color you use in chats (it is a important visual hint). And some cons too: the file transfer is not very stable and the Unix/Linux/BSD client could be updated more often.
I'm looking for a decent alternate Yahoo Messenger client for Linux, as the official one sometimes crashes. I'm trying GaimMessenger, it has some very interesting features and you can program plugins in C or Perl. It has Unix and MS-Windows versions.
Try a JabberProject client (for example, I use Gabber). There are many Jabber servers which act as gateways to the Y!M network (and ICQ, MSN Messenger and so on - I have one client that connects to all my IM accounts).
EveryBuddy is a reasonable unofficial client.