About Free/Open Source applications for Windows

FOSS or F/OSS is a general term for Free/Open Source Software. The term FLOSS is also seen; it stands for Free (Libre)/Open Source Software. Libre is used indicate that "Free", in this case means "free as in speech, not as in beer". Gratis is often used to specify "free of charge". Free (libre) software is almost always gratis.

Free Software and Open Source Software (OSS) are closely related ideas, that are differentiated by some philosophical points; for most end users they are effectively the same.

Free or Open Source Software has various advantages over closed source software, whether it is "freeware" or regular commercial software. FOSS generally does not cost money, although it is often possible to buy support for FOSS, if you need it.

Freeware aka closed-source gratis software is often written and maintained by one person. If they do not want to add features or fix bugs or they do not have time to do so, it is generally impossible to do anything about it. With FOSS, other people can contribute, and if the author or project lead is not willing to accept contributions, the project can be forked and maintained with more liberal policies. This has happened many times in the FOSS community under both friendly and unfriendly circumstances. After a while, one may become clearly dominate or, if there is room in the market, the two may coexist.

Much more about Free Software can be found on the web site of the Free Software Foundation, and more info on OSS is available on the Open Source Initiative's web site.

FOSS Apps for Windows

Cross-platform apps work on Windows and Linux and usually Mac OS X as well.
Portable apps are available in non-installing, USB thumb drive-friendly form from portableapps.com
Most of these apps are on portableapps.com or on The Open Disc.
Also check freshmeat.net for a searchable database of Open Source apps for all platforms.

Big Names

Firefox - Browser - cross-platform, portable -- Firefox 3 Release Candidate 1 is now available
Thunderbird - Email - cross-platform
SeaMonkey aka Mozilla - Browsing and email - cross-platform, portable
OpenOffice.org - Office Suite - cross-platform, portable -- OO.o 3 is in beta, it features support for the new MS Office formats (.docx, etc), a solver for the spreadsheet, improvements to graphing, and improved notes for Writer.
Gimp - Raster graphics editing - cross-platform, portable -- Gimp 2.6 should release this summer with a new core called GEGL, which will bring support for more than 8 color bits per channel, better color space handling, and eventually faster previews and operations and non-destructive editing.

Create

Blender - 3D modeling and animation - cross-platform
Inkscape - Vector graphics - cross-platform
Scribus - Publishing and layout - cross-platform
Audacity - Sound editing - cross-platform, portable

Editors

Nvu/kompozer - Web editor, html and wysiwyg - cross-platform, portable
AbiWord - Light weight word processor - cross-platform, portable -- 2.6 recently released with many improvements
Notepad2 - Text editor (Scintilla engine) -
Notepad++ - Text editor (Scintilla engine) - portable
Scite - Text editor (Scintilla engine) - cross-platform, portable¹

Communication protocols

Azureus - BitTorrent (p2p) - cross-platform
Filezilla - FTP client - cross-platform, portable
Putty - ssh (secure shell) - cross-platform², portable
TightVNC - VNC remote desktop - cross-platform, portable
UltraVNC - VNC remote desktop - cross-platform²
Pidgin - Multi-protocol IM - cross-platform, portable
Miranda-IM - Multi-protocol IM - portable
Psi - Jabber/XMPP IM - cross-platform
RSSOwl - RSS/Atom feed reader - cross-platform, portable
Miro - Video aggregator - cross-platform
gizmo project - Internet voice calling (like Skype) - cross-platform
Ekiga - NetMeeting clone, VoIP (SIP) client - cross-platform³

Fun & Games

Celestia - Planetarium - cross-platform
Stellarium - Planetarium - cross-platform
Wesnoth - Overhead RPG - cross-platform
Really Slick Screensavers - cross-platform

Utilities & Misc.

Sumatra PDF - cross-platform², portable
VLC media player - cross-platform, portable
PDFCreator - cross-platform, portable
7-zip - Compression utility - cross-platform, portable
Clamwin - Antivirus - cross-platform, portable
TrueCrypt - File encryption - cross-platform
KeePass - password manager - portable
GnuCash - personal finance - cross-platform

KDE 4

Most KDE 4.x apps are now available for Windows. This does not yet include the "workspace" aka the shell/Plasma, so you cannot yet use KDE as an explorer.exe shell replacement, like BBlean or Windowsblinds. However, many useful KDE apps and fun games are available. Some larger projects, such as KOffice have their own release schedules, and may not be available for Windows yet.

Dolphin - A basic, user-friendly file manager
Konqueror - A web browser and power user's file manager
Amarok - A powerful music player, with integrated lyrics and artist info lookup, as well as integration with Magnatune, an online music store for independent artists. More online store integration may be coming as well. (2.x runs on Windows, independent release schedule)
Ocular - Powerful document viewer, handles PDFs, PostScript, Open Document Text (openoffice/.odt), images, and other formats
Gwenview - Full-featured image viewer
Kalzium - Chemistry app that includes a periodic table, element information and a chemical equation solver. In the future, it will also feature 3D molecular images
Marble - Desktop globe, less powerful than Goggle Earth, but smaller and more lightweight
KOffice - Office Suite - includes word processing, spreadsheet, presentations, database, image editor, project management, diagram tool (Visio replacement), and more. (2.0 will run on Windows, independent release schedule)

Notes
1 - Scite has a native portable version, rather than using portableapps.com
2 - This program is not cross platform, but programs that use the same protocol or file format are available on other platforms
3 - Ekiga has a beta version for Windows