A list of those programs you only need once in a blue moon but can never remember the name of.
lsof - list all file handles. Can be useful when a filesystem won't unmount.
modinfo - describe module and arguments that can be provided
htop - top on steroids
atop - lists the most active processes based on CPU, memory, disk and network usage
iotop - lists io usage of programs.
duc - visualise disk usage