nginx ("engine x") is the chosen server for roughly 15% of the world's busiest websites. As an HTTP, HTTP reverse proxy, and email (SMTP, IMAP, and POP3) proxy server, nginx is built for smoothly handling demanding traffic with minimal memory usage.
Lintel has used nginx for clients with some seriously demanding workloads. One recent install pushes over 10 million hits a day, and nginx handles it like a champ.
nginx is distributed under a BSD-like license as an affordable and formidable competitor to proprietary / commercial offerings. Following on the heels of Apache, another respected and powerful open source web server, nginix is gaining well-deserved popularity. Whether you're launching your MVP, or scaling the largest of applications, Lintel's recommendation is going to be nginx.