For me some of the following are a must have after I have installed Ubuntu.
Please add links. and a note if the program isn't free (as in freedom)
Update: Collecting all the packages listed below in the answers
Graphics
inkscape - vector based illustration tool
picasa - photo management (non-free)
Gimp - Raster based image editor
Alchemy - Funky little illustration tool
pinta - Paint program
ktoon - 2D animation tool
Dia - Draw diagrams
Fontforge - font editor
Office Tools
scribus - desktop publishing
Lyx - word-processor
Abiword - word processor
syncevolution - contacts * calendar syncing.
Media
guayadeque - music player with auto-playlists using last.fm
vlc - movie player
XBMC - media management
Exaile - media player
banshee - media player with ipod/android device support
spotify - needs wine. Internet media player (non-free)
foobnix - music player
clementine - music player based off Amarok
radio tray - applet to listen to online radio
bad/ugly gstreamer plugins
mplayer - media player
pithos - gnome applet for Pandora
pianobar - command line player for Pandora
Development
rapidSVN - GUI for subversion
ruby - scripting, fun!
python - scripting
subversion - version control
cscope - code browsing
ctags - code browsing
vim - my primary code editor
emacs - editor for some more nice tricks
ack - replacement for grep
geany - code editor
git - SCM
lamp-server - dev environment
build-essentials - Not worth compiling from source without this
rpm build - build rpms
MySQL Workbench
Meld - Diff & Merge
Eclipse/Eclipse CDL - IDE
JDK
gPHPedit
Code::Blocks - C++ IDE
Eric Python - Python IDE
smartsvn - subversion GUI frontend
netbeans - IDE
g++
powerpad
Qt designer - Designs GUIs using Qt
xCHM - Reads CHM help files.
Meld Diff Viewer - Compares differences in files
Gedit Plugins - Makes Gedit a great,stable programmer's editor
Internet
elinks - text based browsing
vpnc - connect to office over VPN
google chrome - web browser (non-free)
chromium - Web browser, free version of chrome
midori - web browser
pidgin - messenger/IRC client
multiget - download manager
filezilla - download manager
deluge - bittorent client
last.fm - client for last.fm website
skype - voice chat (non-free)
putty - terminal client
opera - web browser (non-free)
thunderbird - email client
epiphany - email client
zimbra desktop - email client
dropbox - free online storage and sync across desktop (non-free)
adobe air - internet application framework (non-free)
checkgmail - mail notification
davmail gateway - connect to MS Exchange using clients like thunderbird
Google Earth (non-free)
wireshark - network analysis tool
kismet - wireless analysis tool
gpodder - podcast downloader
gbittorrent - torrent client
GmailWatcher - notifications for gmail
Firefox plugins
vimperator - vim like shortcuts for firefox
tab mix plus - additional tab commands
colorful tabs - makes it easy to identify related tabs
fox clocks - world time
download statusbar - downloads in status bar
xmarks - sync bookmarks across PCs and browsers
ad block plus - block ads in web pages
Productivity
gnome-do - shortcuts for everything in ubuntu
docky - frontend for gnome-do
getting things gnome - task manager
System Configuration
Ubuntu tweak - install packages using this
Ubuntu restricted extras - restricted packages for ubuntu (non-free)
System Tools
wine - windows emulator
virtualbox - OS virtualization
remmina - remote desktop client
crash plan - system backup (local/online, personal/commercial)
burg - boot loader based off GRUB
htop - system resources viewer
AWN - Avant Window Manager
truecrypt - encryption
synergy - Share mouse & keyboard with another PC
ailurus - tweak Ubuntu for newcomers
unetbootin - create bootable USB drives
Bootup Manager - manage boot scripts and services
Accessories
screenlets - desktop plugins
recordmydesktop
terminator - terminal client supporting split windows
caffeine - prevent screensavers and sleep mode
easystroke - gesture recognition application
zotero - collect, manage, cite and share research sources online
byobu - makes GNU screen more useful
keepassx - password management
screen - keep sessions alive
guake - terminal
work rave - prevent RSI, take breaks
mousepad - an editor that loads faster than Gedit
A lot more of these are listed on one of my older blog post, however I am only listing the most frequently used ones here. What works for you? Please list the name and category for the program, as well as what you use it for. I have made this a community wiki.
