Thought I would start an Ultimate Thread on what free software is available for all to use (apologies if one already exists, I did have a quick search for one).
Here's a few of my favourites
Cross-platform:
http://www.mozilla.org - Mozilla main site
http://www.mozilla.com - Mozilla Firefox web browser
http://www.mozilla.com/en-US/thunderbird/ - Mozilla Thunderbird email client
http://www.apple.com/safari/ - fast web browser based on Konquerer and Apple WebKit
http://www.gimp.org - GNU Image Manipulation Program, image / photo editing software
http://www.inkscape.org - InkScape, vector art program
http://www.openoffice.org - OpenOffice, full office suite including word processor, spreadsheet, database, presentation, graphics
http://audacity.sourceforge.net/ - Audacity, sound recorder and editor
http://www.clamav.net/ - ClamAv, Anti-virus software
http://www.dosbox.com/ - DosBox, an x86 emulator with DOS
http://hatari.sourceforge.net/ - Hatari, Atari ST emulator (BYO Roms!)
http://www.mplayerhq.hu/design7/news.html - MPlayer, Movie Player program
http://www.videolan.org/vlc/ - VLC, portable media player and streaming server
http://bellard.org/qemu/ - QEMU processor emulator, can emulate Windows and others quite effectively
http://www.skype.com/intl/en-gb/welcomeback/ - Skype, make free video or audio calls over the internet around the world
http://sidplay2.sourceforge.net/ - SIDPlay play those old C64 tunes!
http://www.viceteam.org/ - VICE, 8-bit computer emulator (C64, Vic20, PET, etc)
http://xoops.widelands.org/ - Widelands, a realtime strategy game similar to Settlers I & II
http://www.sane-project.org/ - SANE, drivers for just about every scanner ever made
Mac:
http://www.bombich.com/software/ccc.html - CarbonCopyCloner, data backup tool
http://www.iconus.ch/fabien/products/pleng/pleng.html - PDFLab, Create and edit PDF files
http://www.barebones.com/products/textwrangler/ - TextWrangler, a good text editor
http://wakaba.c3.cx/s/apps/unarchiver.html - The Unarchiver, works with pretty much any archive file type
http://www.unrarx.com/ - UnRarX, works on Rars that The Unarchiver can't handle
http://www.derailer.org/paparazzi/ - Papparazzi! Web page screen capture utility, captures the entire page in an image file
http://www.kju-app.org/ - Q, a nice port of QEMU
http://www.neooffice.org/ - native port of OpenOffice
I know there are various lists out there already, but this is MY list
If anyone asks if there is software to do xyz then I can point them to this list, although it's not exhaustive by any means. Keep in mind that, as with the Mac ports, there may be specific ports and projects for your particular operating system (e.g. Windows) so it's worth looking at the links and hunting around for these, as they are usually optimised and should run better for you.
Hope it's useful to all you peeps in eBid land anyway! The beauty is that it's all completely freeeeeee!