There is an abundance of useful software available for download gratis. From
web browsers to word processors to graphic manipulation programs, there is
plenty of software available online.
Though most of what is available is safe and useful, its always a good idea
to do a little research and make a backup before installing anything new. The
following are some proven and reliable programs that are available, free (or
gratis), for individual use (and sometimes more, check the license).

Windows
• Web Browser: Mozilla Firefox or Opera (Opera is usually faster on slower
hardware and works better on older versions of Windows).
• E-mail Client: Mozilla Thunderbird
• Office Suite: OpenOffice.org
• Disc Tools: CD Burner XP, Burn at Once
• Instant Messenger: Gaim, Trillian, Miranda
• Media Player: DivX Player, Nullsoft Winamp, iTunes, SnackAmp
• Anti-virus: AVG Anti-Virus, Free Edition, avast!Antivirus Home edition,
ClamWin
• Security: Spybot: Search & Destroy, Ad-Aware Personal Edition, K9
Anti-Spam,ZoneAlarm Free Edition
• Compression: 7-Zip, IZArc, TUGZip
• Desktop Search: Google Desktop

GNU/Linux
Unlike Windows, on a GNU/Linux system the majority of the software that
you will want for everyday use of your computer is usually included. You will
probably not need to download anything. Most GNU/Linux distributions have
a package manager (Portage for Gentoo, APT for Debian-based distros like
Debian and Ubuntu, etc.) For some distributions you can simply download
RPM or DEB files from your distribution’s web site.
If they aren’t already installed by your distribution:
• Web Browser: Firefox or Opera (Opera is usually faster on lower-end
hardware).
• EMail Client: Mozilla Thunderbird.
• Office Suite: OpenOffice.org or KOffice
• Instant Messenger: Gaim or Kopete.
• Media Players: Rhythmbox, mpd, amaroK (depending on what desktop
environment you use), Beep Media Player or Video Lan Client.
• Movie/DVD Player: Xine or MPlayer.
• Windows Compatibility Layer: WINE.
• x86 Emulator: QEMU.
• PPC Emulator: PearPC.