GWS (Google Web Server). Powers websearch, responsible for rendering everything you see when you run a search. It used to have a Wikipedia page but basically everything on the Wikipedia page was wrong so the admins eventually deleted it (edit: apparently it's back. Basically everything on the wiki page is still wrong, but at least it doesn't claim GWS is an Apache derivative now). It's the oldest continually-pushed binary at Google (originally written 1999 by Craig Silverstein; at least when I left it still had code in it from Craig, Marissa Mayer, and Sergey Brin). It's changed programming languages twice (from C to C++ around 2005, and from C++ to Java etc. around 2010), and when I left was a frankenmonster of C++, Java, and two proprietary DSLs. My understanding is that it still exists, though most of my friends at Google have since moved on to other teams.