Based on a somewhat current ubuntu forum thread titled “Top Ten List of Software you need in Ubuntu Linux” the author encourages you to take a look at his article and then comment on what are your top apps. The information below is based on the readers of this thread and their comments or suggestions.

APP NAME VOTES
1 FIREFOX 6
2 GAIM/PIDGIN 5
3 DELUGE 5
4 THUNDERBIRD 5
5 MPLAYER 5
6 GIMP 4
7 CONKY 4
8 K3B 4
9 WINE 3
10 XCHAT 3

More Here.