Essential if you have HDMi audio out AND Headphones/analogue and need to switch at night without going into menus or fiddling around with volumes and knocking your ear drums out ( cinnamon does this already and gnome has a plugin )
Wget : This is a terminal program to make HTTP requests, a popular use case is to simply download the source of a web page or grab a file from a web server in a terminal.
btsscaner : This tool attempts to extract as much information as possible from a Bluetooth device without having to pair.
SET : The Social Engineer Toolkit Social Engineer Toolkit or SET is a framework that makes setting up social engineering attacks easier. Email based spear phishing attacks, fake websites and wireless access points can all be launched through its menu system. In this case they are using the SMS spoofing module.
Here are some of my favourites, I've tried to clean it up, deleting already posted suggestions, apologies if I missed one or two. Also, I'm not running Ubuntu, so some of the programs may be either installed on your system by default or the packages may have slightly different names. Cheers!
GUI urxvt -- plugins for tabbing and quake like terminal jdownloader -- download stuff (from videosites, and from file sharing sites) lyx -- create pdf documents with math formulae qemu -- Virtual machine (mainly for quickly testing live cds) FileBot -- organize your movie/TV-episode collection, and download subtitles
Command Line weechat and irssi -- Best IRC clients in existance (IMNSHO) ncdu -- like WinDirStat, but for the terminal tmux -- terminal muxer (multiple terminal sessions in a single window) lshw -- list hardware lsof -- list open files (figure out what program is using your USB, and many other cool things) apt-file -- find package containing a particular file abcde -- rip mp3 ssh -- remote login rdesktop -- log on to your windows box remotely lilypond for writing sheet music bmon -- view network statistics nmap -- look for computers and services on the network z -- quickly jump to recently visited folders ix -- paste to ix.io
SUPPORT zip/unzip/unrar/p7zip terminus (awesome terminal font) ntfs-3g -- NTFS support exfat-utils -- Support for Extended FAT file system (Windows USB drives)
ADVANCED testdisk/photorec -- rescue a messed up harddisk ddrescue -- rescue a messed up harddisk
You could add Crusader which is Linux equivalent of Total Commander in windows. Its great ffile explorer/organiser. And qTox de-centralized encrypted instant messaging service.