The addons I use are:
Cryolysis2 - A very helpful mage addon that houses all your ports, food conjuring, when you need to buy reagents, mana gems, mount, hearth and you can even set a few up. I would say its my most helpful addon for my mage. De-clutters the interface a lot.
Note: Stick to the 'non-urgent' spells, and items.. I found out the hard way that its not the best place for health pots and stones. And definitely not Ice Block, lol.
X-Perl - Great for modifying your interface. Works smooth and fits well with the game. Also de-clutters interface. And its really good for raiding :) And the 3d moving portraits are pretty nifty XD
Titan Panel - Just a bar at the top and bottom of the screen, displays some useful info :)
Omen - Pretty much 100% of raiding guilds require you to have it. And if you dont have it, your unlikely to get many offers to run instances. It monitors your threat, so you can do maximum dps without pulling aggro.
Deadly Boss Mod - Not really necissary until you are doing raids, and even then, if someone in the raid has it, you wont need it. But it's an expected addon.
Questhelper - Pretty much tells you everything about quests. Where they are, what you need to get, how many you need to get, where you need to hand it in, etc. Very helpful for levelling :)
Ventrilo - Not really an addon, but a lot of higher end instances, heroics and raids will expect you to have it. Its basically a voice chat program. Its a lot better than the in-game one.
And atlas, like you mentioned is also a great mod :)
Hope that helped, happy gaming!