Pitbull = User interface to customize your party/raid
http://wow.curse.com/downloads/details/7738/

IceHUD = Shows your health/mana/rage and those of your target in the middle of your screen as arcs or bars
http://www.wowinterface.com/downloads/full4222.jpg
http://i139.photobucket.com/albums/q...w/ICEHUD02.jpg
Info: http://www.wowace.com/forums/index.php?topic=1705.0

Personally I also like to use:

Automaton: Addon that automizes small things like selling all your gray items when you're at the vendor, automatically repairing, skipping gossip text.
http://www.wowace.com/wiki/Automaton

Multiboxer: Mostly because of the function to automatically refollow your main. Carefull here though, the swapping of action bars caused me some concern, since I use a lot of bars. So I had to comment that part out.
http://wow.curse.com/downloads/details/9618/

JasonQuest: See your twinks questlog with progress as well as their XP and rested. Since my twink windows are quite small, it is sometimes hard to read stuff that is on their window, so now I can see that on my main screen.
http://www.wowace.com/wiki/JasonQuest

WIM: Great Addon for managing tells. You have have several conversations open at once in seperate windows or tabs. Never miss another tell on one of your toons since you can scale the tell window to take up the whole screen.
http://www.wimaddon.com