Digg is running on the followings
- Caching (Memcache)
- Distributed file system (MogileFS)
- Monitoring (Nagios)
- Asynchronous Processing (Gearman)
To know more about the architecture, you can read the followings
Existing eBay website is built using Java. There is a good presentation on the eBay architecture. It is interesting to see the evolution to Perl based script to Microsoft IIS, and the to a custom built Java solution.
YouTube is built using Python, running on Apache under Linux, using MySQL as the database server. More details at YouTube architecture.
Amazon is using a mixture of technologies (Perl, C++, Java, Mason) running under JBoss in Linux using Oracle. Amazon architecture is a fully-distributed, decentralized, services platform serving many different applications. More details at Amazon Architecture.