Sniffer for serial, i2C, SPI, onewire,
RAM usage monitoring ...
Turn the Open Bench Logic Sniffer into a shield
Ability to interpret commands to and responses from slaves.
A lot of the time, simple serial prints are all that is needed,
being able to see the hardware bits flipping is sure handy at times
a tri-state/input mode test would be handy? Maybe pull a pin high/low with something like 0.5mA load, if it can wiggle it is not active output?
most Arduinos don't have JTAG.
I wonder if you can get DebugWIRE working somehow without official Atmel hardware
The four serial inputs could be 2 sets of Rx/Tx pins?