There are many shareware and freeware gems available for the IIGS, some of which exhibit quality beyond a few commercial game releases.
As the Link Turns
Bouncin' Ferno
Bouncing Bluster
Carte Primus
Columns
Cosmocade
DuelTris
Milestones 2000
One Arm Battle
Orbizone
Pangea's 24 Hour Games
Plotting
Plunder
Quest for the Hoard
Sensei
Senseless Violence 1 & 2
Solarian
Space Cluster
The Tinies
Wolfenstein 3D