Here are some of the apps I use and what operating systems they are available for (that I know of).
Word processors (in order of what I use most):- TextEdit (Mac OS X and Rhapsody)
SimpleText (Mac OS 9)
AppleWorks (Mac OS 9 and Mac OS X)
Okito Composer (Mac OS X)
HTML design and editing:- PageMill (Mac OS 9)
HTMLEditor (Rhapsody)
BBEdit (Mac OS 9 and Mac OS X)
Freeway (Mac OS 9)
GoLive (Mac OS 9 and Mac OS X)
Internet Apps:- OmniWeb (Mac OS X and Rhapsody)
Watson (Mac OS X)
Netscape (Mac OS 9 and Mac OS X)
Internet Explorer (Mac OS 9 and Mac OS X)
WebGrabber (Mac OS X)
Mail (Mac OS X)
Outlook Express (Mac OS 9)
MailViewer (Rhapsody)
PopOver (Rhapsody)
Fetch (Mac OS 9 and Mac OS X)
RBrowser (Mac OS X and Rhapsody)
Image/Document editing:- Photoshop/Image Ready (Mac OS 9 and Mac OS X)
Acrobat (Mac OS 9 and Mac OS X)
TIFFany3 (Mac OS X and Rhapsody)
Create (Mac OS X and Rhapsody)
Painter (Mac OS 9 and Mac OS X)
Media Editing (Video/Audio):- Quicktime Pro (Mac OS 9 and Mac OS X)
Strata VideoShop (Mac OS 9)
SoundEdit 16 (Mac OS 9)
SoundEffects (Mac OS 9)
Games:- Quake III (Mac OS 9 and Mac OS X)
Quake II (Mac OS 9 and Rhapsody)
Doom I & II (Mac OS 9 and Rhapsody)
Unreal (Mac OS 9)
Unreal Tournament (Mac OS 9 and Mac OS X)
Rainbow Six & Rogue Spear (Mac OS 9)
Bugdom (Mac OS 9)
Cro-Mag Rally (Mac OS 9 and Mac OS X)