Hi Jaime,
Maybe the ports could be named xymon-server and xymon-client, for minimum ambiguity. Then "xymon" can be either nonexistent (forcing the would-be installer to notice the real names) or even just print out a warning/readme.txt.
Buchan said it might be better to have consistent naming across platforms. Your proposal of using "xymon-server" and "xymon-client" might also apply to other packages than the MacPorts. So RPMs and debian packages might be adapted as well.
I don't care about the names of the packages as long as there's some documentation to guide the users. In the case of the macports it is specified which port is the client/server part on the wiki page: https://trac.macports.org/wiki/howto/SetupXymonServer
And Mac users can search for xymon ports with the following command:
$ port search xymon xymon @4.3.0.beta3 (net) Xymon network monitor Client
xymon-server @4.3.0-beta2 (net) Xymon network monitor Server
Found 2 ports. $
Which clearly identifies which one is the client and which one the server.
As said before I don't care about the names of the packages and therefore I don't see a problem if they change. The only thing is that it shouldn't create myself more work. This is why I propose to people willing to change the names to propose their own Portfile patch (documentation is here: http://guide.macports.org/#project.update-policies) and submit it to the macports community. Even being the maintainer, I don't have commit privileges so you'll need to create a trac ticket (http://guide.macports.org/#project.tickets) to get your changes accepted by the macports commiters.
Regards, Francois.