I run a monitoring page for NTP servers.  If you want more info on NTP, you should go to http://en.wikipedia.org/wiki/Network_Time_Protocol. If you’re wondering whether ntpdate is good enough to keep your clock synchronized, don’t. It isn’t. Just go install ntp (or ntpd) and let it run.

If you’re interested in joining the pool, the NTP Status page is a good resource for finding reliable upstream servers (the official page still has some dead servers, despite my efforts).