Linux is a awesome operating system with unlimited potential due to being open source, flexible and well designed. Hidden within Linux is a treasure trove of commands that even some Sr. Administrators may not know about. I have listed some of these below:

#1. dmidecode – dmidecode will give you essential information on hardware in the system. a example of this being the need for bios manufacturer and version information should you need a potential bios update.
Example:

# dmidecode -s bios-vendor && dmidecode -s bios-version
Phoenix Technologies LTD
2.1

#2. nohup – This allows you to run a command that is immune to hangups

#3. head -n1 /etc/issue – displays the current operating system. You can also cat /etc/redhat-release or /etc/debian-version

#4. cat /proc/partitions – Will display all registered partitions in the system.

#5. rlogin – this comand will connect to a remote host with the same username and password you are logged in with. rsh will also do the same thing.

#6. dosfsck – fsck DOS partitions

#7. tracepath – a simplified version of traceroute

#8. shred – overwrites a file making the contents unrecoverable then deletes it.

#9. warnquota – emails users who are exceeding their disk quota

#10. swap on and swap off – These two commands will enable and disable your swap file.

#11. mpg123 – Play mp3′s via command line – program must be installed.

#12. hdparm -tT /dev/sda – does a disk speed test

For a whole lot more check out http://www.oreillynet.com/linux/cmd/