I have gathered together a starter list of Free and Open Source Software programs for the K-12 classroom that you can start using in your classroom right away--and that can be given to students for free.
Audacity -
http://audacity.sourceforge.net/
Audacity is a free, easy-to-use audio editor and recorder for Windows, Mac OS X,
GNU/Linux, and other operating systems. You can use Audacity to:
Blender -
http://www.blender.org
Blender is the Open Source Software for 3D modeling, animation, rendering,
post-production, interactive creation, and playback. Blender allows students and
teachers to animate 3D computer graphics.

Celestia -
http://www.shatters.net/celestia
The free space simulation that lets users explore the universe in three
dimensions. Celestia comes with a large catalog of stars, galaxies, planets,
moons, asteroids, comets, and spacecraft, as well as a catalog of additional
downloads. Students and teachers can plot a course and navigate a 3D solar
system. See also
http://www.stellarium.org
Dia -
http://live.gnome.org/Dia
Dia is inspired by the commercial Windows program 'Visio', though more geared
towards informal diagrams for casual use. Teachers and students can use it to
draw many different kinds of diagrams.
FreeMind -
http://freemind.sourceforge.net
FreeMind is a premier, free mind-mapping software written in Java. Teachers and
students can use mind map diagrams to represent words, ideas, tasks or other
items linked to and arranged radially around a central key word or idea. It is
used to generate, visualize, structure and classify ideas, and as an aid in
planning, organization, problem solving, and decision making.

GIMP -
http://www.gimp.org or
http://www.gimpshop.com
GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of
software for such tasks as photo retouching, image composition and image
authoring. GIMP works on many operating systems, in many languages.
See also
http://www.inkscape.org
Moodle -
http://www.moodle.org
Moodle is a course management system, designed using sound pedagogical
principles, to help educators create effective online learning communities. It
can be downloaded and used on any computer (including webhosts), yet it can
scale from a single-teacher site to a 50,000-student university. We're calling
it a desktop program since you access Moodle using the web browser, but it does
need to be installed on a server somewhere. We will have our own moodle
server in the district starting with the 2007-2008 school year.

OpenOffice.org
-
OpenOffice.org
OpenOffice.org the product is a multi-platform office productivity suite. It
includes the key desktop applications, such as a word processor, spreadsheet,
presentation manager, and drawing program, with a user interface and feature set
similar to other office suites. Sophisticated and flexible, OpenOffice.org also
works transparently with a variety of file formats, including those of Microsoft
Office, and the vendor-neutral OpenDocument standard from OASIS.
Scribus -
http://www.scribus.net
Scribus brings award-winning professional page layout to desktops with a
combination of "press-ready" output and new approaches to page layout.
Underneath the modern and user friendly interface, Scribus supports professional
publishing features such as CMYK color, separations, ICC color management, and
versatile PDF creation.

TheOpenCD -
http://www.theopencd.org
The Open CD is actually not a program, but a very easy-to-distribute collection
of popular free and open source software programs, including most of the titles
mentioned so far, and many more. You download the .iso image file, and can burn
and distribute this as a single CD for both educators and students to install
these programs.

Tux Paint -
http://www.tuxpaint.org
Tux Paint is a drawing program for children ages 3 to 12 (preschool and K-6). It
combines an easy-to-use interface, fun sound effects, and an encouraging cartoon
mascot who guides children as they use the program. Kids are presented with a
blank canvas and a variety of drawing tools to help them be creative.

Ubuntu -
http://www.ubuntu.com
Ubuntu is a full operating system, like Windows or the Mac OS, which can run
either as a "Live CD" or can be installed fully on the hard drive. Ubuntu has
really come of age and is rapidly becoming an incredibly useful alternative when
looking for a robust, easy-to-use, free operating system. There is a special
version for education community (includes new thin-client LTSP) at
http://www.edubuntu.org, and another version for old computers called
Xubuntu at
http://www.xubuntu.org.
KNOPPIX -
http://www.knoppix.org
Knoppix is actually a full operating system with a collection of programs, and
is downloadable as a .iso CD image file. KNOPPIX is the technician's best
friend, and actually runs as what is known as a "Live CD," meaning that you can
boot KNOPPIX from your CD ROM drive and it doesn't affect (or need) the PC's
hard drive at all. KNOPPIX gives you an incredible variety of utility and
recovery programs for troubleshooting and solving PC issues.