Useful Commands is a list of commands that do useful things to monitor your system. Generally these are Debian commands, but are not limited to that.
Get Power On Date
Monitoring (with other system stats)
List Ram usage per case
List block devices (hard drives, flash drives, CD's, etc)
List first level disk usage:
du -chxd 1 | sort -h
Monitor Network Traffic (src, dest, speed, rate, total data, etc)