Audacity - http://audacity.sourceforge.net
Audio Editor

Firefox http://mozilla.com/firefox
Web browser

Foxit Reader - http://foxitsoftware.com/pdf
PDF Reader

Free Mind - http://freemind.sourceforge.net
Mind Mapping Software

GeoGebra - http://geogebra.org
3d Geometry/ Algebra Software

GIMP - http://www.gimp.org
Photoshop - like graphics editor

Inkscape - http://inkscape.org
Vector Graphics Editor

italc - http://italc.sourceforge.net
Classroom Computer Manager

Komposer - http://komposer.net
Web-Page Authoring System

Scribus - http://scribus.net
Desktop Publishing Software

Stellarium - http://stellarium.org
3D Astronomy Mapping Software

Thunderbird - mozillamessaging.com/thunderbird
Email client

VLC - videolan.org/vlc
Portable media player

Zimbra - http://zimbra.com
Email and Calendar server