Here are the main reasons I maintain and enjoy OS 9 machines...

for graphics and animation...

Photoshop 7

Illustrator 9.0.2

After Effects 4.1

Premiere 5.1

for programming...

BBEdit Lite 4.6

MPW 3.5

Power Mac Debugger 2.2

ResEdit 2.1.3

HexEdit 1.9.2

MacErrors 1.2.1

for online stuff...

Classilla 9.2.2

Fetch 3.0.3

miscellaneous apps & utilities...

ClarisWorks 3

SoundEffects 0.9.2

Pixel Spy 1.2.2

The Tilery 3.1

Not to mention the excellent games. For starters...

Alone in the Dark trilogy

Warcraft II

TIE Fighter

Day of the Tentacle

Indiana Jones and the Fate of Atlantis

Full Throttle

Marathon trilogy

Flashback

Prince of Persia 1 & 2

Tomb Raider

A-10 Attack + Cuba

MacSokoban

Shanghai

Tetris Max

Eric's Ultimate Solitaire