The NTP Pool is a vital but unappreciated Internet service. It runs quietly providing correct time to millions of computers all on a volunteer basis. A lot of the credit goes to Ask Bjørn Hansen who's been coordinating the project since 2005.
NTP is an old, simple protocol that lets one computer set its clock by the time on other computers. We take it for granted now, but before NTP was widespread computer clocks were often days off correct time (or worse, 3 seconds ahead). Microsoft and Apple both build NTP into their operating systems now and run their own NTP servers, but where do all the Linux servers get their time?
Mostly, Unix servers use the NTP Pool, a surprisingly small set of servers contributed by volunteers. Today there's just 2792 NTP servers giving time to an estimate of 5–15 million clients. NTP itself is amazingly efficient; one packet every 17 minutes once things have settled down. But the pool requires a lot of extra management, tracking reliable servers and providing a fairly complex DNS setup.
I love projects like this, where a small team can build a public resource that makes the Internet better.