xdm is the graphical login program for X.
Normally, machines advertise that they are "Willing to manage" your
display, which isn't particularly interesting since they wouldn't be
on the list if they weren't. This patch will allow Linux machines to
show their Load average in the Chooser, so users can choose the
machine with the lowest load.
The patch is for XFree86 version 3.3, but it shouldn't be hard to patch other versions of xdm.