On Linux Server Documentation

The following information about a server are what I usually take note of:

1. Purpose
2. Linux distribution version (lsb_release -a)
3. Kernel version (uname -a)
4. Processor information (cat /proc/cpuinfo)
5. Memory information (cat /proc/meminfo)
6. Disk partitions (fdisk -l)
7. Network configuration (cat /etc/network/interfaces)
8. Hostname (cat /etc/hostname)
9. Hosts file (cat /etc/hosts)
10. DNS Servers (cat /etc/resolv.conf)
11. Allowed users/Sudoers  (cat /etc/passwd; cat /etc/sudoers)
12. SSH server configuration (cat /etc/ssh/sshd_config )
13. Firewall/IPTables settings ( iptables -L )
14. APT sources ( cat /etc/apt/sources.list ) 
15. Nagios settings
16. Backup process (depends on the solution being used: rsync, bacula, etc. )
17. more to follow...

If you know the specific function of the server, say a MySQL database server, document the configuration (/etc/my.cnf).

Don't forget to backup the config and the data!