I use Trillion, which allows IM/chatting to all four of the main clients: MSN Messenger, ICQ, Yahoo and AOL.
unfortunately, trillian is intended for end-users, not site owners wishing to provide a workable chat service to visitors. the jabber suite of protocols is open-source and the server stuff is free - keeping site administrators from becoming beholden to any one of the commercial IM providers. [having a site that demands you install yahoo chat or AIM or MSN looks, in short, 'cheap' (in the sense of poorly operated, chintzy, clicheed, etc).] digging around on the jabber site a bit should turn up some PHP and other web-friendly clients that you should be able to drop into your CMS or other site software without a tremendous amount of trouble. elijah