Nginx is a free open source web server, reverse proxy and IMAP/POP3 proxy server.
Everyone and their dog talks about WordPress performance these days and also tells you how to scale WordPress (Read: How to really scale WordPress).
By now all of are aware how Mansoor Peerbhoy has masterminded the terrorist attacks in India, that he showed no remorse in killing non-muslims and doesn't care about the consequences, you are also aware how he was a senior software engineer working at Yahoo / Zimbra and earning an impressive salary by Indian standards.
nginx [engine x] is a high quality, light footprint (much ligher than Apache HTTPD), high performance HTTP server and mail proxy server written by Igor Sysoev.
You can configure Nginx to stop referrer spam by checking for bad keywords like tramadol, phentermine etc.