Pidgin allows for a very large number of protocols as well, it's open source (big in many businesses these days), it's plugin-based and it's multi-platform. If you have any clients using Linux, steer them towards the client built in to their distro of choice. Any using a Gnome based distro should at least try out Empathy as it has many advantages. For example, Empathy can be used without opening a full chat window when using Gnome-Shell.
For both of these, Gtalk is built in.

Skype is simply too insecure. It's fine for use at home but should not be brought in to an enterprise environment where security is at all important.