Addons

Quartz:

To look at your real latency and adapt your gameplay to it. (e.g. shortcut a cast for moving without cancelling it)

Rogue spammer:

To spam without annoying UI messages.

Omen:

To track my threat during the fight ! A good mage is a mage alive !

Recount:

To see what shame you can actually give your mates' mages ! And more seriously it's and incredible tool to know if you are under-performing and if you need to wake up and focus.

Needtoknow:

It displays your procs on your UI ! It shows their timers as bars ! That gives you more visibility on timing your CD usage properly.

Tellmewhen:

Useful for looking at buffs or debuffs on you, or on the target. You can also track you cooldowns.

Mage nuggets:

I only use it to display my stats in live on my UI. It's useful to verify that you have the maximum of SP possible when you use your Combustion.

Sexy cooldown:

Track your CD and ICD in real-time. I prefer using Heatsink for it.

Snowfallkeypress:

Trigger your spell when you press your keys and not when you release them, saving you some time.

Heatsink:

I use it to track my ICD.

Decursive:

To dispel magic and curses on my mates when it's needed. Works for all classes.

Cooldowns:

To see my cooldowns on my bar.

Ora3:

To track raid Cooldowns.

RangeDisplay:

To improve your placement during Encounters by showing you your range from different targets on Mouseover, Actual Target or Focus.

DBM:

I use DBM only for timers, I prefer using Tellmewhen for harming debuff, or other "need-to-move-fast-things", it's less messy.

Besides these addons I use some UI addons such as Bartender and Tidyplates to improve my UI.