Here is the list of apps I use almost everyday on my Debian GNU/Linux computer :
Dropbox is my favorites. The fastest. However Wuala and SpiderOak offer better security
A very good color picker
Dynamic DNS client
I use it with namecheap.com
The best way to memorize
By far the most complete IRC bouncer
Those are the best IRC client. I prefer WeeChat but irssi is awesome too.
IRC to IM gateway
I use it to access Twitter and Facebook from my IRC client
Beautiful and fast MPD client
chkrootkit is very nice
I use it to capture the whole screen and region
Zsh shell is awesome. Especially the completitions features.
Simply the best!
Conky is awesome, beautiful and powerful
Simple and sweet
The most advanced terminal multiplexer
I use Vim almost everytime but it still useful to have gedit in some case
Tiling Window Manager
The best Window Manager
Firefox will always be my favorite browser
cowsay displays a cow (and other character) that thinks or says what you want. You can combine it with fortunes or figlet/toilet. eSpeak voices are better than festival voices. linuxlogo obviously show an ascii linux logo. Synergy share your mouse and keyboard between many computers.