grep
less
vim
most
ssh
nmap
ping

mc
moc
joe
finch
irssi
ssh ;D
newsbeuter
alpine
elinks
of course, all the nice utilities, find, grep, wget, etc.
bti
rtorrent
bsdgames ;D

IRSSI
Mutt

Most frequently used:

man
grep
less
mount/umount
find
make
yum
which

Favorite:

ffmpeg (totally awesome!)
man
stunnel
clamscan

I have quite a few! I actually blogged about 8 of them that I like:

iftop, atop, lsof, netcat, watch, jumpgate, pstree, strace

http://www.linux.com/myblog-admin/enkryptik-observations-8-cli-tools-that-are-under-utilized-sometimes-unknown-or-unappreciated-and-yet-pack-a-powerful-punch.html

Cheers - Kryptikos