OS X menubar/dashboard/dock utility for monitoring a server?

Solution 1:

GeekTool (free)

GeekTool will let you set up a simple icon or show more detailed information, embedded in or floating over your desktop, based on whatever programming you want.

If you can write a script to check the server and return either a binary status or more detailed information, GeekTool is the way to display it.

Solution 2:

ServerSiren ($15 with 10 day free trial)

With Server Siren, server downtime is a thing of the past. It runs as a menu bar item, so a quick glance is all it takes to see that everything is A-OK. If a server goes down, Server Siren alerts you immediately so you can fix the problem and get your site up and running again.