My favorite is the software he built that YC alums get to use.
From 2008-2012 we had an email list.
Garry wrote a Rails site (IIRC) in 2012 and that was a clear inflection point for YC alums. Total game changer for many of us and has provided lots of value over the years. I think it was also the starting point for online SUS.
Congrats Garry/YC!