DuckDuckGo is coded in Perl and JavaScript with the help of the YUI Library, served via nginx, FastCGI and memcached, running on FreeBSD and Ubuntu via daemontools. We both run our own servers and have servers on Amazon EC2 across the world. Your connection generally goes to the closest regional server available to your area. We use PostgreSQL+bucardo, CDB, Solr, Berkeley DB, Amazon S3 and flat files for data. We monitor via Server Density, our own scripts and DNS Made Easy (which we also use for DNS and failover). Finally, we have some side components that use Prosody, Debian, ejabberd, jQuery, node.js, Python and even more Perl with lots of CPAN distributions.

Also check out this architecture post for a more in-depth look at how we function on the backend.