Pidgin is the default instant messenger client in Ubuntu, and was formerly known as Gaim. With Pidgin you can talk to people who use AIM/ICQ, Gadu-Gadu, GroupWise, IRC, Jabber, MSN, Napster, Bonjour and Yahoo, and list all your buddies in a single window.

To start Pidgin, choose Applications → Internet → Pidgin Internet Messenger.

There are several extra packages available for Pidgin, which add extra features and allow you to change the way Pidgin looks. You can find these in the Synaptic Package Manager.

Ekiga is the default Internet telephony (VoIP) application in Ubuntu. It lets you make audio and video calls over the Internet to people who have hardware or software that follows the SIP or H.323 standards and also features basic instant messaging. It is compatible with Windows Messenger and most VoIP applications. However, Ekiga cannot communicate with Skype because the latter uses proprietary technology. If you have the right SIP provider, you can also use Ekiga to place calls to any normal phone line.

To start Ekiga, choose Applications → Internet → Ekiga Softphone.

See the Ekiga Manual for help on using Ekiga.

For users of IRC instant messaging services, an alternative to Pidgin is XChat-GNOME. Its features include advanced scripting support (perl and python) and a clean, easy to use interface.

One excellent way to get help with Ubuntu or to introduce yourself to IRC is to join Ubuntu's official support channel on Freenode.