Buddy lists
The most familiar means of displaying presence information is the buddy list on services such as Google Talk, Microsoft Live Messenger and AIM - , a list of a user's contacts and thier current status e.g. Busy, Online, Away etc.
Presence Technology
What is Presence?
Presence Information is an indication of the ability and willingness of the users of a communication network to engage in an exchange of information. Knowing the presence status of a user is essential to many types of IP based communication such as VoIP and Personal Instant Messaging.