- arduino-max – Using an Arduino as an input device controller for Max/MSP, providing 12 digital and 6 analog inputs. See also the Max wiki page.
- eeprom_programmer – A EEPROM programmer.
- max395 – Controlling an infinite number of digital switches!
- radiosphere – Owen Cartwright's use of Max395 chips to control power to 80 clock radios.
- psx – Using a PlayStation controller as an input device.
- red_on_black – Controlling an alphanumeric LCD display.
- Use a DAC chip, such as the National DAC0802 8-bit D-to-A converter. Some quick observations: Funky power requirements. Lots of external passive components.
- Wow, the Analog AD8300 looks bitchin.
- It just gets better: Analog AD5665. Serial input. Four 16-bit DACs.
As many digital outputs as your heart desires
- We've all been using and enjoying the Max395 chips. I've got a library of code on svn. 
- Current record: Owen with 80 individually addressable switches. Radiosphere using a slightly older version of the Max395 code.
Lots of external storage
- Look into interfacing to an SD card.
- Maxim technote on interfacing a microcontroller to an SD card.
- SD socket, $18.
- Wiring up a socket to run on 5V (the SD sockets need 3.3V). Here. Plus some stuff about the FAT16 file system on SD cards.