These are two pics of an unfinished, and heavily modified, multiboxing UI that I did eventually finish:


Eventually the action bars hid themselves out of combat and Forte Exorcist would show my CDs among other tweaks here and there [like removing the extra redundant totem bars]. The list of addons was [going off of memory]:
Code:
Pitbull
Dominos
Prat
Combuctor/Bagnon
beql
SexyMap
WinterTime
Fortress
DurabilityFu
FriendsFu
GuildFu
MoneyFu
kgPanels
Postal
Quartz
ISBoxer Click Bars
Satrina Buff Frames
TipTac
Dual Speccer
SLDataText
PreformAVEnabler
pError
caelnameplates
Clique
Mapster
InFlight
OmniCC
Fontain
GhostPulse
Grid
VanaKOS
WIM or Cellular
ZoneDefense
I'm sure there were a few more. As for which ones are useful... well I guess that's all up to the person who wants to mod their UI.