DuckDuckGo is coded in Perl and JavaScript with help of YUI, served via nginx, FastCGI and memcached, running on FreeBSD and Ubuntu via daemontools.  We both run our own servers and have servers on EC2.  We use PostgreSQL+bucardo, CDB, Solr, BerkelyDB, S3 and flat files for data.  We monitor via Server Density, our own scripts and DNSMadeEasy (which we also use for DNS and failover).  Finally, we have some side components that use prosody, Debian, ejabberd,jQuery, node, 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.