Hi. My name is "Jayar" and I am addon addict.

MANY addons are a personal taste sort of thing, so don't be afraid to experiment.

Jamba
Multiboxer Gold. It automates a tremendous number of things. There are modules I don't use, and modules that I turn off because I have other addons I prefer for those functions, but the core MB bits are wonderful. Being able to push configuration info to the slaves is wonderful.

Smartbuff
I like Smartbuff and I have the "smartbuff key" macro'd into my "Follow Me" key, so everytime I form up after a combat all my toons refresh any missing buffs.

Unitframes (many available)
I use Pitbull, but it has a bit of learning curve. X-perl and Shadowed unitframes offer much of the same core functionality and are a bit easier to configure out of the box. The important parts: Ability to arrange and move frames (as a multiboxer you will probably become pretty particular about where things that are clicked need to be) Heal prediction/estimation.

Button Bar Replacement
This is less important for multiboxing because you end up doing less clicking, however I enjoy having control over where the buttons are placed and the ability to hide and show them as I want. I use Bartender. Macaroon is much more powerful but requires a more fiddling to get "just right".

Auction Addon
Auctioneer is the Grand Dame. It does a thousand and one things and has some very advanced models built into to its analysis functions. It can also be a bit of a pig when it comes to resources. I use auctioneer on my bank toon and Auctionlite on one of multibox toons. I am considering dropping auctioneer altogether, but I've used it since the dawn of time, so I haven't dropped it yet.

Map Addon
I weep for death of Cartographer. I am currently using Mapster, but it is a weak replacement for my dear friend Cartographer. As I understand it, there are modules (handy notes?) that you can add to Mapster to get some of the functions back.

Gathering
One of the downsides of Multiboxing is that you can only "track" one resource at a time. One way around that is get a gathering addon that allows you to import data files. Gatherer and Gathermate are the two big kids on the block. You put them on your "driving toon", import the historical data files and turn on herbalism and mining nodes. This isn't perfect, but it makes gather a little bit easier.

I also like Omen, Lightheaded (on my main) an inventory/bag management addon.