Internet 3: Communication

Mozilla Thunderbird
Mozilla Thunderbird

Addons for Thunderbird:
Lightning:
Lightning for Thunderbird

Provider for Google Calendar:
Google Calendar for Thunderbird

Trillian Basic (free)
http://ceruleanstudios.com/

Miranda IM (os)
http://www.miranda-im.org/

Pidgin - formerly known as Gaim (os)
http://sourceforge.net/projects/pidgin/

Skype (free)
http://www.skype.com/download/

Teamspeak
Teamspeak

os: open-source; free: freeware [mostly for non-commercial use];
trial: evaluation version, mostly limited to 30 days;
share: shareware [1. free to use with all features for a short period of time, or 2. free to use with limited features - or nag-screens(winzip)], both kinds have to be paid to become the full versions