Using Instant Messaging
First: General information on Instant Messaging. (TechTV)
Instant Messaging clients have a growing number of features and IM services are gradually beginning to interact with one another. Instant Messaging clients usually make money through small advertisements in the main chat window but many are advertisement-free.
While most IM clients depend on what your friends are using, so much development is happening in this area, its wise to look at several.