Spark is an open source, cross-platform instant messaging client for businesses and organizations. All the SMPP features are supported by the client, including group chats, VOIP, and strong security. You can customize the program with your own skins or modify the config files for corporate customization.
I guess I missed the point that the client was designed to work in a corporate or business environment. When you first run the program, all you can see is the main window with a limited set of options. You can either login or register for an account. To do so, you will need to provide information to an XMPP server provided by your organization. Of course you can go to the same route as I did and find an open server somewhere on the Internet, but what's the point of doing so if you want your connection to be secure?
Jive Software also works on Openfire, an easy to set up and administer XMPP server. The server and the client go hand in hand and let your organization to create a secure environment for exchange of information between employees.
The client has a couple of predefined skins. Some of them look good, others are hideous. There are a lot of settings in preferences (that's the nature of open source software - everything is customizable). Overall, this is not a bad alternative to consider when thinking of switching your company from Skype to a private network IM client.