Mods I suggest people get:
Auctioneer, it tells you the average price items sell for in the ah and automatically puts a good price, it also lets you post multiple stacks of items all at once, without having to manually type in price each time. it scans the ah as you use it and gets the averages for items, so you know basically what everything sells for.
KTM threat meter, not too important now, but when we start raiding everyone should have it, it shows meters on how much threat you have, so you know when yourabout to pull aggro.
Natur enemy cast bar, it shows bars on how much time is left on sheep, sap, ice trap, and any other crowd control ability and it shows enemy cast bars right next to your char so its impossible to miss, it also shows the cooldowns and how much time is left on them on any player you have targeted.
Atlas loot, this shows every item dropped by every boss in every instance, and their drop rates, and it also shows all the faction rewards from every faction, and it shows pvp rewards... basically every item you ever wanted is on here and it shows where to get it.
I dont have the links on hand for these mods, but if you just go to google, type in World of warcraft mods, the second link down, cursegaming.com has all these mods, in the most popular section.