I get asked all the time to recommend free or open source software to replace commercial ones. Most of the time it has to do with the cost but sometimes it's the philosophy. I've started this thread to discuss (and link to) these alternative applications. Feel free to post some of your favorites here and I'll add them to the list.

Anti-Malware
This section includes Anti-Virus, Anti-Spyware, Anti-Rootkit etc...
AVG - Free version of a commercial virus scanner.
Avira - There is a free version for home/personal use.
Spybot Search & Destroy - Free Antil-Malware utility (primarily spyware and rootkit detection and removal)

Chat & Messaging
Pidgin - A multi-protocol Instant Messaging client that allows you to use all of your IM accounts at once (AIM, MSN, Yahoo!, Google Talk,  IRC and many more).
savIRC - An Open Source IRC Client written in Tcl/Tk

Defragmentation
Ultradefrag - This one is open source
Auslogics Disk Defrag - Freeware

Data Destruction
Eraser - Open source (GNU/GPL)

Hardware Diagnostics
memtest86 - A free memory diagnostic utility.

Image Manipulation
The GIMP - 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. It works on many operating systems, in many languages.
Gimpshop - A modified version of The GIMP that makes it look and feel more like Photoshop.

Media Players
VLC Player - Plays just about any type of media file and is open source.
Foobar2000 - Strictly an audio player and freeware

Office Suites, Word Processors, Spreadsheets, Documents
Abiword - An award winning open source word processor
Foxit Reader - Foxit Reader 2.3 is just released. The new version 2.3 includes a multimedia player feature allowing Foxit users to not only read PDFs, but to watch and listen to the audio/video elements coming with PDFs as well
OpenOffice.org Suite - Replaces Microsoft Office and even opens and saves in the office formats. OO.org is open source.

The list is growing. Feel free to post your suggestions

excellent link Fizz. I just updated the list with that software

Comodo BOClean : Anti-Malware V4.26 (freeware)

Comodo BOClean is protection against - Internet trojan horse programs, spyware, keyloggers, rootkits, pseudorootkits, hijackers, adware, email relays, spam proxies, spam relays, scam downloads and email/spam robots, its features - Destroys malware and removes registry entries does not require a reboot to remove all traces disconnects the threat without disconnecting you. Generates optional report and safe copy of evidence Automatically sweeps and detects INSTANTLY in the background Configurable "Stealth mode" completely hides BOClean from users. Optional central administration prevents user tampering Protects itself from malware tampering or shutdown, daily malware database updates from the Comodo web site. Full spectrum malware coverage and protection. Comodo BOClean sits in the background emitting a constant low level scan, has a very small footprint (File Size: 1.65 MB and is very light on resources), please read more at site.
 
Download Link and please note the installation instructions > Comodo BOClean v 4.26

Audiograbber 1.83 (freeware)

(Quote)
Audiograbber copies the audio digitally-not through the soundcard-which enables you to make perfect copies of the originals. It can even perform a test to see that the copies really are perfect. Audiograbber can also automatically normalise the music, delete silence from the start and/or end of tracks, and send them to a variety or external MP3 encoders. Screen shot and download link below.

Click on image to enlarge



Download Link > Audiograbber 1.83