[1] http://www.codersatwork.com/
On the science/research side, his work with Lua has explored language interoperability in a methodical way. He also brought about the first real-world implementation of a register-based virtual machine and gave an apples-to-apples comparison of its merits (Lua 4 to Lua 5). His research also led to the resurrection of the popularity of asymmetric coroutines and parseable expression grammars (PEGs).
From an engineering standpoint, Lua is one of the cleanest languages ever made and it has impacted the video game industry as much as John Carmack (in different ways). The register based virtual machine white paper Roberto wrote was also the inspiration of Apple's Squirrelfish Javascript VM, which kicked off a tidal wave of Javascript VM wars.