I'm not a huge Instant Messaging person, but when I do happen to get online, I prefer to use my Mac's built-in iChat application. It's clean, works great, and is fully integrated with mail and my address book. The only problem is that iChat only supports it's own protocol along with AIM's, and Jabber's. Using a Jabber account, I could get myself into my MSN account, but it was a huge pain in the ass to setup. I'd never been able to find a Jabber server that had Yahoo! transports. So, I just kinda passed on it.