I use a bunch of addons! Here's a few:

PitBull3 - All unitframes except raid
Grid (+Clique) - For raid frames and healing with Clique (not for multiboxing).

Dominos - All visible actionbars
Macaroon! - Hidden actionbars (Storage), only multiboxing related keybinds and macros

Classtimer - Target buffs/debuffs and short player buffs
Elkano's Buffbars - Player buffs/debuffs and HoT tracking
Power Auras - Buff/debuff/proc tracking

HitsMode - Improved combat log, also displays DPS after combat
Mik's Scrolling Battle Text
Quartz
Jamba
NiceDamage
Omen3
Recount
XLoot
Deadly Boss Mods
etc.
etc.
etc.
....