Do it.

This command does a tally of concurrent active connections from single IPs and prints out those IPs that have the most active concurrent connections. VERY useful in determining the source of a DoS or DDoS attack.

Displays a calendar.

Uses the fabulous Hpricot library to parse HTML from Ruby. Extracts all elements of "command' class and displays unescaped text from inside these elements. The following command can help install dependencies (apart from Ruby itself)

Search manpages for a keyword. Very useful when you don't know where to find the information.

Display a passive popup during seconds. Additionnaly, --title can be used to set the title of the popup. This is a nice way to communicate with a desktop user of a machine you have an SSH access on :

DISPLAY=:0 sudo -u $user -H kdialog --passivepopup "Hello you" 10 --title "cli IM"

Very useful when the ssh key of a host has changed and ssh refuses to connect to the machine, while giving you the line number that has changed in ~/.ssh/known_hosts.

This is the simple revision number on stdout, that can be fed to any useful/fun script of yours. Setting LC_ALL is useful if you use another locale, in which case "Revision" is translated and cannot be found. I use this with doxygen to insert my source files revisions into the doc. An example in Doxyfile:

FILE_VERSION_FILTER = "function svn_filter { LC_ALL=C svn info $1 | grep Revision | awk '{print $2}'; }; svn_filter"

Share your ideas about what to do with the revision number !

Modify the script for your username and password, and save it as a script. Run the script, and enjoy ./tweet

Useful for a cron job that runs nightly, gzipping or alternatively deleting files from a specific directory that are older than 10 days (in this case)

Useful to detect which process is causing system loads. It shows process PID so as we can take further actions.

Very useful for finding the largest files and subdirectories at any given point. Any user can run it from current location just when need to know their largest files and subtdirectories from a certain point down as well.

check your load with top... Start more of these jobs to get an multi-core cpu busy...

Simple use of ImageMagick identify function.

This is just one method of checking to see if an IP is blocked via IP tables or CSF. Simple and to the point. Replace xx.xx.xx.xx with the IP you wish to check.

Very useful for finding all files over a specified size, such as out of control log files chewing up all available disk space. Fedora Core x specific version.

Read 32GB zero's and throw them away.

How fast is your system?

Should run on any system with ssh installed.

Watch is a very useful command for periodically running another command - in this using mysqladmin to display the processlist. This is useful for monitoring which queries are causing your server to clog up.

More info here: http://codeinthehole.com/archives/2-Monitoring-MySQL-processes.html