Some open source softwares [windows os] - must have

Web Browsing

Mozilla Firefox

The premier free, open-source browser. Tabs, pop-up blocking, themes, and extensions. Considered by many to be the world's best browser.
Download Page

Video Player / Video Podcasting

Miro

Beautiful interface. Plays any video type (much more than windows media player). Subscribe to video RSS, download, and watch all in one. Torrent support. Search and download from YouTube and others.
Download Page

IM - Instant Messaging

Pidgin

Connect to multiple IM accounts simultaneously in a single app, including: AOL IM, MSN, and Jabber.
Download Page

E-mail

Mozilla Thunderbird

Powerful spam filtering, solid interface, and all the features you need.
Download Page

RSS

RSSOwl

Solid cross-platform RSS client.
Download Page

Peer-to-Peer Filesharing

Cabos

A simple, easy to use filesharing program. Gnutella network.
Download Page

Gnucleus

A very good Gnutella search and download filesharing program.
Download Page

Azureus

A powerful, but complicated BitTorrent client. So many features you'll lose your mind.
Download Page

Video Playback

VLC

Plays more video files than most players: Quicktime, AVI, DIVX, OGG, and more. Pretty good interface.
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MPlayer

Similar to VLC-- plays loads of video formats.
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Media Player Classic

Compact, but powerful media player. Plays anything under the sun. No install necessary.
Download Page

DVD Ripping / Video Conversion

Media Coder

Great tool for ripping CDs, DVDs, etc and converting between tons of video formats.
Download Page

Word Processing / Office Suites

OpenOffice.org

Big, full featured suite of tools for word processing and spreadsheets. Compatible with and a free replacement for Microsoft Word documents. Also supports OpenDocument Format.
Download Page

AbiWord

A word processor. Leaner and quicker than OpenOffice. Compatible with Microsoft Word documents and OpenDocument Format.
Download Page

Podcasting

Juice

Solid podcasting client.
Download Page

DVD Ripping

Handbrake

DVD to MPEG-4 ripper/converter.
Download Page

Sound Recording

Audacity

Simple sound recording tool.
Download Page

Graphics / Photo Editing

GIMP

Photo editing application that rivals Photoshop in features.
Download Page

Paint.NET

Graphics editor with a very nice interface.
Download Page

Inkscape

Vector graphics application.
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FTP / SFTP

Filezilla

Excellent FTP program.
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IRC

X-Chat 2

IRC client.
Download Page

Archiving

PeaZip

Fantastic archiving utility to extract and pack archives of an extensive amount of formats. PeaZip also features password protecting archives, keyfile support, system integration, a lovely interface, and much more.
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Note Taking

Freemind

An innovative application for note taking and outlining. Takes time to get used to, but some people swear by it.
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Jarnal

Open Source note taking application written in Java
Download Page

Keynote

Open Source note taking application, very good. I use this myself extensively
Download Page

Personal Finance

TurboCASH

Simple, easy to use application for managing personal finances.
Download Page

Quasar

Easy to use Accounting package
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Chat Clients

aMSN

MSN Messenger client written in TCL/TK aMSN Messenger

X-Chat

IRC Client X-Chat

More Web Browsing

K-Meleon

Fast, nice web browser.
Download Page

Amaya Browser for Windows

W3C developed web browser and editor for multiple platforms
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Screensaver

Electric Sheep

Immersive, trippy screensaver.
Download Page

More Filesharing

Frostwire

GNUTella client based on the Limewire codebase.
Download Page

Also see: Poisoned - Azureus

3D Graphics and Modeling

Blender

Powerful 3D Modeling software.
Download Page

Also see: Kerkythea (illumination engine) - Persistence of Vision Raytracer - Quesa

Stars and Outer Space

Celestia

Open-source 3D planetarium software. Akin to traveling through space (outer space).
Download Page

Stellarium

"Renders realistic skys in realtime." See what you would see if your city wasn't so light-polluted. Used in real planetariums.
Download Page

Also see: macORSA - Orbit Reconstruction, Simulation, and Analysis

Games

Marathon: Aleph One

Engine for Bungie's free FPS series Marathon. Play online, solo play with rich original story, many total conversions and thousands of maps.
Download Page

Zombies

3D zombie strategy game.
Download Page

Armagetron Advanced

3D Tron game.
Download Page

BZFlag

First-person tank game.
Download Page

Battle for Wesnoth

Fantasy / Strategy Game
Download Page

Also see: Snes9X (SNES emulator), Frozen Bubble

Free File Hosting

CC Publisher

Host Creative Commons video or audio files for free on archive.org with this easy uploader.
Download Page

Security Tools

NMap

Easy to use port scanner for Windows
Download Page

Wireshark

Network Protocol Analyzer, formerly called Ethereal
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TrueCrypt

Creates encrypted virtual hard drives for extra security
Download Page

ClamWin

Open Source Anti-Virus. Very good at detecting viruses. Can be plugged into hMailServer for e-mail protection LIMITATION: Real Time scanning not implemented yet
Download Page

Eraser

Delete files securely off your hard drive utilizing DoD standards
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MP3 and Audio Tools

MP3 Gain

Easy to use MP3 manipulation tool for Windows. adjusts sound levels on MP3's so they are all the same
Download Page

MP3 Book Helper

This program provides rapid Mp3 (ID3v1, ID3v2) and Ogg/Speex/Flac Vorbis tags editing and file renaming functions. Mass Tag editing is done using special variables and Regular Expressions.
Download Page

musikCube

Open Source MP3 Player, ripper and organizer written using the .NET Framework
Download Page

Utilities

Cobian Backup

Backup software.
Download Page

7-Zip

File archiver.
Download Page

Developer tools

XEmacs

The classic, super customizable editor, for Windows.
Download Page

KompoZer

Easy to use, open source WYSIWYG html editor
Download Page

SharpDevelop

Open Source IDE for the .NET Framework. Utilizes the multiple programming languages and Windows forms
Download Page

Notepad ++

Open Source programmers notepad for Windows
Download Page

SciTE

Source code Editor for Windows and X
Download Page

Web Developer Server Suite

Very good WAMP Stack (Windows, Apache, MySQL, PHP) Very thouroughly tested on Windows Server 2003 and Windows 2000 Server (preliminary testing on Longhorn Server underway). Highly Recommended.
Download Page

Mapping Tools

Nasa World Wind

Mapping tool similar to Google Earth for Windows. Must have the .NET Framework installed.
Download Page 

 

Other

Graph (graph plotting software)

Startup Manager (pick what you want to launch on startup)

freeotfe "on-the-fly" transparent disk encryption program for PCs and PDAs. 

 

source: http://www.opensourcewindows.org