Hopefully not bumping the thread, creator hasn't updated in awhile, but there's a lot of stuff to add to the list.
Browser:
Flock , a social web browser.
http://flock.com/
Maxthon , add on web browser, kind of like Mozilla I guess?
http://www.maxthon.com/
Camino , a web browser for Mac which I hear is pretty good. (the browser)
http://caminobrowser.org/
Security
Uhm, not to sure about this one, so maybe I'll post something later once I look into it more..
Office Apps
Neo Office , which is Open Office for the Mac.
http://www.neooffice.org/neojava/en/index.php
Audio/Video:
Winamp , which is, just an amazing media player.
http://www.winamp.com/
Songbird , open source music player
http://getsongbird.com/
Instant Messaging:
Not much I can say here, except for the obvious ones like Meebo.
Image Editing:
Bledner , open source creation suite, pretty good for all the major OS'es I believe
http://www.blender.org/
Inksacape , open source graphics editor, similiar to Adobe
http://inkscape.org/download/
Programming:
Nothing here..
Miscellaneous:
Not sure what should be here but,
PortableApps, if noone has said it yet,
http://portableapps.com/
RocketDock
http://rocketdock.com/
ObjectDock
http://rocketdock.com/
ManyCam , HIGHLY RECOMMENDED if you're trying to live stream or record with your capture card. It really helps.
http://www.manycam.com/
AviScreen , screen recorder capturer, similiar to HyperCam but free
http://www.softpedia.com/reviews/windows/AviScreen-Classic-Review-60434.shtml
And well, operating systems
There's JoliCloud now from the Unix kernel, something to look at.
http://www.jolicloud.com/
That's what I have. If I've repeated anything that the previous posters have already said, I'm sorry.