I do a lot of testing on Arduino libraries and sketches. Being able to quickly observe and fiddle with ALL the pins makes this much easier.

I wanted a board that shows the state of all I/O pins on LEDs with all of these properties....

Works for Digital Input, Output, and Input Pullup

Pushbutton to drive the pin high, very weak pulldown (much weaker than on-chip pullup)

Works for analog pins (thumbwheel pot), but also works when the same pin is used for digital

LED shows actual analog level - LED current should be linear to actual voltage

LED current does not interfere with analog voltage

LED does not interfere with weak signals, or input pullup or pulldown

Inexpensive - under $0.50 per pin - many parts are ok, as long as they're cheap!

More details on my DorkbotPDX project blog...