<p>Does anyone know of a similar product to Citrix Server that'll run on the Mac OS?</p>

<p>Essentially, I'm looking to allow multiple remote users to log in to the same OSX Server at the same time (with full visual desktop, not SSH).</p>

OS X's Quartz window server has no remoting abilities, unlike
its predecessor
. X11
does
, but 'native' OS X applications don't use that; of the few Mac apps typically run in X11 (such as GIMP or CrossOver), none are specific to the Mac, so you might as well run them on a different OS.
That said, if all you want is to visually remote-control a session, it is possible to use VNC or a derivative, such as Apple Remote Desktop. Since 10.4, this allows for multiple simultaneous sessions,
as implemented with Vine Server
. Remote Desktop also has other abilities such as remotely installing and updating software.
(Unlike Citrix and X11, VNC does
not
send drawing commands over the network; it instead transmits a compressed image representation pixel-per-pixel.)
You should specify your exact needs.
You will not get a Citrix-like experience where you can run single Mac apps in their own remote session.
You will, however, get remote graphical control, and that
may
be more than enough for you.