geoMAD by Paul Murdaugh is a neat little program that draws a picture of Alferd E. Neuman line by line, so you can watch it happen in realtime.  The original program came from issue 258 of MAD Magazine (October 1985) and was a program listing for BASIC 2.0 on the Commodore 64.  Paul Murdaugh re-wrote the program using geoBASIC for all of us GEOS nerds to enjoy and show off to our friends.

geoMAD Prime by Paul Murdaugh - After doing some more digging on the GEOS group over at Facebook, I ran across another version of the MAD Magazine (Issue 258, October 1985) program for GEOS on the Commodore 64 that Paul re-wrote in geoAssembler.  This version is 100x faster than the BASIC 2.0 version in the magazine.  There is also a geoBASIC version Paul wrote that you can find here.

GEOS Desktop 9.9 [v1.0] by FlatDisk Software (Dated 6/20/1991) is a geoPARODY by Rich La Bonte.  It really is quite comical, worth trying out at least once ;)  Includes the geoBASIC source code if you're interested in some of the techniques.

GeoSID v1.0 by Scott E. Resh (October 21st, 1986) - GeoSID is a music player that can play .MUS type SIDs.  There are a few sample SIDs in the archive to test things out.  

I don't really have a category for GEOS sound related apps, so I'm putting this in the Novelty category until I run across more apps of this type.

Magic Mouse v1.0 by Milton Johnson (March 3rd, 1991) - This is a neat little application that turns the the pointer in GEOS v2.0 into a cute animated mouse.

Meltdown! v1.1 by Spike Dethman (Dated February 13th, 1989) is a GEOS novelty.  When run, this desk accessory slowly melts the screen... very neat.

Termites! v1.1 by Spiek Dethman (Dated February 13th, 1989) is a neat novelty program for GEOS and the Commodore 64.  When run, this program slowly eats away the screen as if it had termites.