Anyone who has ever looked at the Koha code will have noticed there is one module everything else uses, C4::Context. Back in September of 2002, this was not the case until Andrew wrote this neat module.
It has been poked at by a lot of people since then, but 269 lines of it are still the lines that Andrew wrote. It still retains the purpose it was written for to provide a single place to get things like config variables, system preferences and to implement database connection pooling.
With 163 commits to his name this was not all Andrew did, but it is probably the one that has had the longest lastest influence.
If you read this, stop by #koha on irc.katipo.co.nz and say hi sometime.