o Amiga mod player .:. http://cocomod.stalkingwolf.net/ .:.
[Sadly no Deliplayer on OSX but hey do you remember the Intro to this one? :O) http://spielwiese.marune.de/_uni/_tmp/Bamiga_Sector_1_and_Cybertech1.mod ]
o SIDPLAY .:. SIDPLAY - the premier C64 music player for the Mac .:.
[Although it doesn’t sound as warm and wonderful as the SID in hardware it’s still great. From a psychodelic experience… http://spielwiese.marune.de/_uni/_tmp/Necromancer.sid ]
Cyberduck for FTP: http://cyberduck.ch/
Blender 
Audacity for sounds: http://audacity.sourceforge.net/
Colloquy for irc chat: http://colloquy.info/
Free games: numerous 
Where to begin…
Adium - MSN/AOL/Google IM
VLC - Misc Video Playback (some stuff quicktime won’t do well, even with plug-ins)
Wings3D - 3D Modelling
Smultron - Code Editing
Quicksilver - Greatest thing ever (if you don’t know about it, go to http://quicksilver.blacktree.com/)
Quinn - Procrastinating Tool
BonEcho - Web Browsing (speedy Firefox)
Webkit - Web Browsing
Twitterific - Twitter
SoundSource - Input Managing
Cyberduck - FTP
Colloquy - IRC
CocoaMySQL - Database Management
I guess a better question for me would be, what non-free software do you use?
AGhost
Oh and I am pretty sure a large % of people here use firefox… Interesting no one even thinks of it as “free software”. Its just there, like the rocks and the trees 
A really good browser is www.flock.com. It’s basically FireFox but with a nice RSS reader built in.
…games:
o kaboom .:. http://spielwiese.marune.de/_max/kaboom/index.html .:.
[ppc only but still fun for me]
o gridwars2 .:. http://spielwiese.marune.de/_max/gridwars/index.html .:.
[same here]
o ScummVM .:. http://www.scummvm.org/ .:.
[For playing most of the best adventures]
o Hi-Toro .:. http://www.pimley.net/projects/ .:.
[UAE :O)]
o Frodo .:. http://www.versiontracker.com/dyn/moreinfo/macosx/12590 .:.
[C64 emulator]
o MacMAME .:. http://www.macmame.org/ .:.
[Although it’s way behind the win version]
o VisualBoy Advance .:. http://vba.ngemu.com/ .:.
[gb/gba emulator]
o Nestopia .:. http://www.bannister.org/software/ .:.
[snes emulator]
Audacity
Quicksilver
http://www.blacktree.com/
Pixen
http://opensword.org/pixen/
[Pixel based art program]
- Great Thread -[/color]_[/b] “Should Be A Sticky In External Tools”
TextWrangler
Colloquy – irc client
Aptana and Eclipse if you’re into that kind of thing
Camino, FireFox, Opera
Celestia
Democracy (net TV)
Flip4Mac (lets QuickTime play WMVs)
HandBrake (DVD ripper)
MacMAME (Arcade Machine Emulator)
OSXII (Apple II Emulator)
Wings3D
Blender
But frankly, I think it’s the cheap software that makes OSX awesome. Cheetah 3D, TextMate, etc.
o HEX Editor - PeekIt .:. http://www.ravenware.com/ .:.
[Good for Peeks and Pokes]
o Audio Converter MAX .:. http://sbooth.org/Max/ .:.
[Although slow on lowend systems]
o Font Explorer X .:. http://www.linotype.com/fontexplorerX .:.
[Font Management]
o UnRarX .:. http://unrarx.sourceforge.net/ .:.
[Extract RAR files]
Shark and OpenGL Profiler… 8)
…I’m not adding XCode because it still has a long way to go before it’s great.
Guys please also provide links for the apps you find great and where you think they could be of use for others as well. Personally i would have found such a list very helpful when i first came from win to osx and i’m still learning about great little proggies…
Font Explorer by Linotype beats Apple’s own font mananagement tool and even Extensis Fusion by so much it’s funny.
Quicksilver. Fantastic.
o Micon .:. http://www.filewell.com/micon/ .:.
[Change your Icongfx]
o Cog .:. http://cogosx.sourceforge.net/ .:.
[OGG, MP3, … Player]