I've made a couple of ChucK apps for it since I got it:
A simple usage of LiSa in combination with the monome makes a great fun samper/sequencing experience.
The accordeome is a kind of MIDI keyboard with a layout that facilitates playing chords.
The note sequencer is one of those sequencer apps that you instinctively make on the monome - though I made a piano roll like thingy instead of sample grains or parts. I think it works fairly ok.
I made a digital Clock, that displays on the monome's buttons.