Every Macintosh running OS X is able to act as a time-server for the rest of the network. <shameless_plug>As described in the book Unix/Linux Survival Guide in detail, doing so is quite easy.<shameless_plug> You just have to add the following lines to the file ntp.conf which usually resides (on a well-groomed Unix-System) in /etc:
…
server 127.127.1.0
fudge 127.127.1.0 stratum 10
This approach should work on every *NIX-based system, just remember, that Apple tends to overwrite the file when applying patches from time to time. Check the book for an exhaustive explanation.