There are five files (other than databases) four that are critical and one that is highly desired to the individual server admin and should be backed up daily and secured close tot eh server should need arise to restore a server or otherwise make a critic response at the console.
File names below are generic but should help you find the files in your server(s).
notes.ini, servername.id, log.nsf, names.nsf
If you keep current copies of these files close to the server, the server itself will be quickly restored. However, the databases themselves could fall victim to attack and can be very messy as well especially of there is alot of replication in your environment.