There are a number of competing web servers but nginx is probably your best bet, conceived relatively recently in 2002 by Igor Sysoev, nginx was built from the ground up with the explicit goal of handling a large number of concurrent connections. In simple terms, whereas Apache creates a new process for each new connection (visitor/http request) nginx offloads requests to worker processes and allows for asynchronous resource allocation. What this means in practice is a much faster, more efficient server environment and in turn a faster website.
Sounds great, right. But how would you go about setting this up? Luckily, the nginx & WordPress experts over at rtcamp have build an awesome script (easyengine) for taking a stock Ubuntu VPS image, installing the required server components and a fresh WordPress install with pre-configured support for caching, in basically 3 system commands!