General list of apps I use on my Ubuntu desktop:

thunderbird – great email client
banshee – quite possibly the best music/media player out there
vlc – this can play any movie you throw at it
transmission-daemon – great for running transmission when you aren’t logged in to the machine
ushare – unpn/dlna media server – works great with xbox 360
filezilla – personal favorite ftp client
pidgin – empathy sucks
gnome-do – this helped me transfer finger memory from OS X
gparted – I am a storage guy – I format and look at things.
ssh – is there anything ssh can’t do?
ubuntu-restricted-extras – youtube is boring without flash
xchat – for when I’m feeling extra creepy
vpnc – great cisco compatible vpn client
network-manager-vpnc – makes it guiful
ttf-droid – I love the android font
audacity – I cut up mp3s too
terminator – awesome terminal app