This weekend Bit Crusher, 4 Channel Mixer, and the 303 will be on sale for 99 cents. The 4 Channel Mixer's ability to use insert effects is particularly useful. Simply drop an effect on it and everything will be wired up!
The 303 and Bit Crusher also go great together. It's easy to achieve a classic distorted acid base line. Connect them up and explorer.
Check out this tribute to the 303 on YouTube.