Great thread idea. I have a few contributions:
A3 belly pan
5Q0825236Q
^ I don't have that personally, but it's supposed to fit and has the duct for transmission cooling. May need some extra hardware to install? Good to protect oil pan and tranny from light rocks, dirt, and salt in winter climates.
Someone already mentioned the jack pad adapters, but FYI there are some extra Audi-stamped trim pieces that work for the fronts so you don't have to cut the existing trim with a hole saw:
Underbody Cover - Front Right -->
8V0825272
Underbody Cover - Front Left -->
8V0825271
This is what they look like:
And then these covers go over the pads themselves:
Front Jack Socket - Right -
8V0802846
Front Jack Socket - Left -
8V0802845
All very reasonably priced on e-acca