Trillian is a great Instant Messenger that's fully compatible with all the other major services: Windows Messenger, AIM, ICQ, Yahoo, Skype, MySpace IM, Facebook, Twitter and more. The key advantage is that you only need one login for all these accounts and it's an ideal way of bringing together various friends and business colleagues, across multiple IM accounts, within one user-interface.
In order to communicate with users on these IMs it's necessary to have accounts with the appropriate ones. If you already have accounts these can be entered during set up and all existing contacts will be imported into Trillian's address book.
The interface is effective, enabling you to bring in different themes. Some may be way over the top and for business users, you may miss the simplicity of simple interface that just does its job.