AtomTime98
AtomTime98 is a Windows Internet (Winsock) which will connect you to a time server. The program will compare the time of your PC and the server time that atomic precision and shall adjust the clock of your computer if necessary. There are many options to automate everything and make the whole process transparent. AtomTime98 also supports proxy servers.

Dimension4
Dimension 4 is one of the best time synchronization utility for PC (Windows 95/98 and Windows NT 4.0).

Tardis
This shareware offers the ability to synchronize its clock to a LAN or the Internet in a very friendly.

Vremya
Developed by Kirill Levchenko, Vremya is an application that adjusts the clock on your Macintosh, making use of NTP (Network Time Protocol). Vremya requires System 7 or later, and MacTCP or Open Transport. It is also advisable to have the dashboard Planisphere configured correctly, and the daylight saving time option dashboard Date & Time if any. Vremya is a software for free use.

Tardis K9
Linux version offers the ability to synchronize its clock to a LAN or Internet

Timesync server
Xntp, A set of tools to synchronize your machine under Linux. enter : ntpdate canon.inria.fr and your PC will be synchronized. You have to be connected to the Internet.