Mozilla Firefox - a fast, open source, secure, and extendable browser from Mozilla. Offers a number of features over Internet Explorer, including Smart Location Bar, Places Manager, and a rich database of extensions.
Opera - Feature-rich browser. Includes email client, BitTorrent downloads, Speed Dial (page with quick access to favorite websites), and mouse gestures.
Safari - Ported from Mac OSX, Apple brings Safari to Windows. Features private browsing and other enhancements.
WORDPAD
OpenOffice - a Java based office suite with features that rival the commercial Microsoft Office, and is compatible with Microsoft’s own formats.
AbiWord - Word processing program similar to Microsoft Word. While it does not include all the applications of OpenOffice, it can serve as a replacement for Wordpad.
NOTEPAD
Notepad++ - a feature-rich text editor. Features syntax highlighting, function completion, tabs, and a plethora of other capabilities.
Notepad2 - While not as feature-rich as Notepad++, it offers a simple replacement for Notepad with syntax highlighting.
CALCULATOR
Power Calculator (XP only) - Microsoft offers a free more powerful alternative to the built in Calculator.
COMMAND PROMPT
CygWin - CygWin emulates the Linux shell for Windows, allowing you to work with a more powerful command line.
BUILT IN UNZIPPING UTILITY
7-Zip - Supports a number of formats, including RAR, ZIP, TAR, and 7z; can also generate a self extracting archive.
TUGZip - Supports a wide range of formats and can create a self extracting archive.
WINDOWS MEDIA PLAYER
foobar2000 - While only audio, supports a wide range of formats without plugins, and has low memory usage and is extremely customizable.
iTunes - Media player from Apple. Provides access to the iTunes Music Store (although not recommended as it bundles DRM with songs).
Songbird - In alpha stage, integrates well with music websites. Supports syncing to other devices.
VLC Media Player - Does not have a library feature, but plays almost any media format.
Miro - Video only, features RSS downloading, BitTorrent, and support for video websites such as YouTube.
WINDOWS MEDIA CENTER
XBMC for Windows - Supports a large number of formats and is skinnable.
Elisa - Provides support for a large number of media formats, external devices, auto detection of media on the network, and basic Internet integration.
PAINT
GIMP - Advanced image editor with functionality similar to Adobe Photoshop.
Paint.NET - While not as advanced as GIMP, Paint.NET provides a large feature set, including support for layers and unlimited undo.
WINDOWS SEARCH
Google Desktop Search - Allows you to search the computer and Internet history in an interface similar to Google Web Search. Also provides a sidebar and quick launch utility.
Copernic Desktop Search - Provides an intuitive UI for searching and claims to be light on resources.
ADD/REMOVE PROGRAMS
Revo Uninstaller - Powerful uninstaller that attempts to remove all traces of a program when uninstalling.
DISK CLEANUP
CCleaner - Searches system for unneeded files and cleans them. Also has a registry cleaner, start up manager, and minimalistic Add/Remove Programs interface.
REMOTE DESKTOP
TightVNC - TightVNC sets up a server on the computer you want to control, and client software on the controlling computer. Offers a number of features over Remote Desktop.
UltraVNC - Like TightVNC, with support for a Java viewer, text chat, and encryption.
TeamViewer - Quick setup, with support for both Windows and Mac. Has file transfer; no firewall setting changes needed.
WINDOWS PHOTO GALLERY / WINDOWS PICTURE AND FAX VIEWER
Windows Live Photo Gallery - Microsoft provides an enhanced version of Windows Photo Gallery free, as part of the Windows Live suite. Provides photo library tools.
IrfanView - Light image viewer with a wide range of features, including image manipulation and conversion.
XnView - Another light image viewer with a wide range of features. Image manipulation, conversion, and meta data editing are a few features.
WINDOWS MOVIE MAKER
Jahshaka - Offers much more functionality than Windows Movie Maker. Open source.
LiVES - Not available for Windows natively, but can boot from a Live CD. Has a greater feature set than Windows Movie Maker.
WINDOWS MESSENGER
Windows Live Messenger - Offers more functionality (and bloat) than Windows Messenger. Supports only the MSN protocol, with some integration with Yahoo.
Pidgin - Supports a wide range of protocols, including Yahoo!, MSN, IRC, AIM, Google Talk, Jabber, and others. Extendable with plugins.
Digsby - Supports a wide range of protocols as Pidgin does, and also features integration with social media such as Facebook.
Miranda IM - Light messenger with support for a wide range of protocols. Highly extendable.
Yahoo! Messenger - Offers some integration with the MSN protocol. Mainly supports the Yahoo! protocol.
WINDOWS EXPLORER
xplorer2 - Offers a wide range of features over Windows Explorer, including dual pane and tabbed browsing.
Blackbox for Windows - An alternative light shell replacement for Windows, based off of the Blackbox Window Manager. Highly customizable.
DISK DEFRAGMENTER
JkDefragGUI - Is able to defrag when your screensaver comes on. Has other powerful features.
OUTLOOK EXPRESS / WINDOWS MAIL
Mozilla Thunderbird - Email client from Mozilla. Extendable with plugins and themes.
Windows Live Mail - An improved version of Windows Mail, Windows Live Mail is part of the Windows Live suite.
Evolution - Originally for Linux, Evolution is now available for Windows, offering an Outlook-like GUI.
Mail functionality is also provided by SeaMonkey (formerly Mozilla) andOpera
WINDOWS FIREWALL
Comodo Firewall Pro - Protects outbound connections as well as inbound connections, unlike Windows Firewall. Has database of “safe” software to prevent unnecessary warnings.
Audacity - Record, mix, and apply effects to sounds in this fully featured audio editor.
CD/DVD BURNING FUNCTIONALITY
ImgBurn - Works with all types of media, able to create, build, and burn CD images.
WINDOWS SIDEBAR
Google Desktop Sidebar - Google Desktop features a sidebar feature as part of the Google Desktop package.
Yahoo! Widgets - While not a sidebar, Yahoo! Widgets allows you to place widgets onto your desktop. Also features a widget dock where you can view your widgets at a quick glance.
Klipfolio - Place “Klips” around your desktop, with information from the web. Takes up less space than traditional widgets/gadgets.
START MENU
Vista Start Menu - Larger start menu which allows you to find your applications more easily and quickly perform tasks.
Launchy - While technically not a menu, Launchy allows you to launch programs by typing a portion of the application name.
ViStart - For XP users, this replaces the current Start Menu with Vista’s start menu, even containing search functionality.
RUN
Launchy - Launch programs by typing a portion of the application name - as opposed to typing the full application name or executable name.