Here is my list of the greatest Programs ever written:
1- The Linux kernel:
Not only does it power well over half the internet, it also powers 80% of the mobile devices on the planet (Android), as well as many consumer devices (TIVO, many routers, etc)
The foundation on which the Linux kernel is based. GCC is a key component of the GNU toolchain. The Free Software Foundation (FSF) distributes GCC under the GNU General Public License (GNU GPL). GCC has played an important role in the growth of free software, as both a tool and an example.
While not popular themselves, they kicked off this whole World Wide Web thing and transformed the Internet from an academic curiosity to a basic utility that drives a big part of the economy.