I am particularly fond of the Latch module. It is basically a flip flop that switches whenever the voltage goes high at its input. Normally, you'd use it to press a button to switch something on and have it stay on until you press the button again.
Because it switches once for very two times it is triggered, it makes a handy clock divider so you can have one synth voice triggering at half the tempo of your sequencer clock. Here is an example: vimeo.com/97028344
If you add the latch after an oscillator, the tone will be down one octave so it creates a fat sub bass sound without carefully tuning two oscillators.
If you have the microphone module, you can use it as a guitar octave divider too (works better with distorted guitar because the waveform is more square).
The microphone module is the one I'd buy first though, as it opens up a universe of fun processing live instruments and other audio signals.