StackShare uses HAML
Personally, I really like HAML. Not having to use open and close tags is a huge time saver. As a result, writing markup with HAML is much more pleasant. HAML essentially forces you to be very strict about spacing, organization, and structure. It also makes the markup easier to read. Protip: I use this pretty frequently: htmltohaml.com
papaver uses C
been programming in c for over a decade, since learning it in college. still use it for various low level projects. used it recently to develop an embedded application for a custom board.
micro systems uses C
The core of the arcapos applications is written in C, so are most of the Lua modules (bindings to various hardware or protocols).
Sqreen uses C
The Sqreen PHP agent is both a PHP extension, built in C, and a daemon built in Python.