Ordered by usefullness to me

Bindpad
I use xkeys and this is sooo nice because I can match the layout easily.. it's very light weight on slaves over macaroon memory wise and it does not mess with keybindings when I switch a slave to main to play for raids on another pc. My multibox experience improved a lot when I added this and I dumped bartender3 on slaves (would have converted to macaroon after 3.0 patch).

Jamba
This is turning out to be a great consolidated multibox mod. I have run into some bugs but I have confidence they will be fixed. Going to stick with this one for sure and let me delete several other mods that do simular things.

Keepfollowing (Jamba does this? haven't tried it yet, but this one works great!)
I used to follow by pressting a /follow macro. This replaced that and is so reliable. Only thing that sucks about it is it breaks drinking, but you can do a special in game macro to work around it, but I rather just not have it follow when you start drinking instead.

Macaroon (Main).
This Addon is amazing and very powerful. Unlimmited buttons and does not seem to mess with keybindings when I switch PC's as long as I use the macro version (simular to Bindpad). The thing I like about it most is on Pally tank can put Hand of Salv or BoP right next to the character bar (pitbull)... so when a slave gets agro I just click the button right next to their name. You can even bind the bars to only show up if you have a target, certain party member is in group, combat, etc. Overall I like it MUCH better then bartender which I have used for a long time.

Auctioneer (light version with just Appraiser, not the suite... WAAAAY faster)
I had the suite for ahwile and it was so slow... I mostly used it cause I had huge stacks of items after running several heroics (shards, dust, etc) and wanted them to split and easy to post. Also for jewelcrafting this is a must have since you are always price matching (appraiser). I tried the lite version which has appraiser and it was at least 10x faster searching and posting auctions... Much better now.

Decursive
Have seperate wireless mouses on 2 other PC's used to click stuff (Mages on one, priest on other). This is super easy for both mage and priest to clear curses, box turns red, click, goodness.

Fubar (different colors for each character)
I mostly use this for performance fu (like to see the lag meter) and the fact I can have custom colors for each mage. Colors is much easier to quickly glance at then processing their names in the brain, especially in the morning.

Pitbull (again different colors in the Player health bar that matches the fubar color)
Same as fubar, really like how you can have a custom background color for the player health bar and make it really big, makes it much easier to glance at status, resizeable, movable, etc. On my main the buff tracking and castbars of the slaves is a MUST have and the fact I can make them really big to see what spell they are casting at all times and how long it is till the spell hits, their targets, etc.

ZOMGBuffs
One button buffing. Has some bugs but overall it's useable.

Recount
I always monitor the DPS of the mages to check their specs and if they are working properly (do a lot of heroics).

OpenAll
Do a lot of AH stuff. Like being able to push button and "automate" looting of the mail.

Smartqueue
- Great for automated AV joins, leaving bg's, etc...

SimpleMiniMap
Resizes the map. I make it smaller on most but very large on Gatherer slave so I can see the dots easier since that window is usually in the PiP.