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
Perljobs.Ru uses Perl
The whole backend part (deployment and other scripts, business logic, web interface) is written in Perl.
ΠΠ΅ΡΡ Π±ΡΠΊΠ΅Π½Π΄ (ΡΠΊΡΠΈΠΏΡΡ Π΄Π΅ΠΏΠ»ΠΎΡ ΠΈ ΠΏΡΠΎΡΠΈΠ΅, Π±ΠΈΠ·Π½Π΅Ρ-Π»ΠΎΠ³ΠΈΠΊΠ°, Π²Π΅Π±-ΠΈΠ½ΡΠ΅ΡΡΠ΅ΠΉΡ) Π½Π°ΠΏΠΈΡΠ°Π½ Π½Π° Perl.
jagalbraith uses Perl
I use Perl to rip through log files and compare them to some signature files I have created. When I get a match, it adds the bad guy to the list of shame in MySQL.
Provide Booking uses Elixir
Huge boon to productivity when coupled with Phoenix. Moreover, it has made background jobs and all the unseen aspects of a business easily abstracted.