Messenger for Mac
Like most IM clients, Messenger revolves around text chat. What makes this program unique however is its catering to corporate users. When matched with Office Communications Server 2007, the program restricts talk to co-workers and authorized Messenger, Yahoo and AOL Instant Messenger (AIM) users, as well as iChat users with an AOL e-mail account. Essentially, a company can manipulate IM networks so that employees don't contact strangers and/or risk a malware infection.
Besides corporate accounts, the program also naturally lets you create personal ones. Unlike the former though, which can chat with AIM users, personal accounts can only chat with users on Live Messenger or Yahoo Messenger.