The "OCF machine status" web page is generated by a Perl CGI script that reads most of the same data as the "ruptime" shell command (thus the script's name). For efficiency, the OCF uses rwhod++ to collect this information with queries from a small number of servers; it's then made available over NFS. There's also code (no longer used at the OCF) to figure out whether X terminals are in use by looking at information compiled by the ICSI version of the finger daemon. For systems that don't run the rwho and rstat RPC services (e.g., Windows) the script uses ping.
The icons were drawn by hand using the GIMP, in standard sizes of 64x64 for machines with flat cases and 64x96 for machines with tower cases.
The source code is freely available:
ruptime.pl (as of Sep 28 2003)
FingerInfo.pm
You can see all the present and past icons (and borrow them, if you wish) here.