At a time when many of us are working with thousands of machines, Paul Tyma provides a remarkable example of how much you can do with just one.
Paul runs the clever Mailinator and Talkinator services. Mailinator lets people receive e-mail to arbitrary addresses under mailinator.com, mostly for disposable e-mail for avoiding spammers or annoying registration requirements. Talkinator is an instant messaging system for easily setting up and joining chat rooms.
Both are examples of removing the login friction normally associated with an application (in this case, mail and instant messaging) to discover a new application with different properties and uses.
In an older post, "The Architecture of Mailinator", Paul describes how he optimized the single server running the service to handle 6M e-mails/day. To summarize, the system is custom-built to the task, all unnecessary goo removed, and focuses on robustness while accepting a very small probability of message loss.
Recently, Paul updated that older post. Now they are processing 2M e-mails/hour on a single machine, 3.1T of data per month, and might have to expand to a second machine, not because the one machine cannot handle the load, but because two machines is the cheapest way of getting the extra bandwidth they need.
Paul also posted details on the architecture of Talkinator, which also is highly optimized to run well on a single server. If you take a peek at that, don't miss his amusing second post where he tested running on a 5W tiny plug-in server.
I am not advocating spending as much effort as Paul does on minimizing server costs. But, his work an inspirational counter-example to the common tendency to throw hardware at the problem. It is an enjoyable tale of building much by staying simple, both in the application features and the underlying architecture.