First Addon and (on my Favorite) is Auctioneer;
Download it at: Auctioneer addon for World of Warcraft
I recommend AuctioneerAdvancedSuite if you enjoy detailed information on bid, sales, and many other gadgets like telling you what items will disenchant into, and if your an enchanter, use the enchantrix barker to advertise prices! (i do not recommend scanning the auctionhouse on WoW anymore then once every other day for an Accurate reading.)
I will also recommend Informant, and Bottom Scanner: Both from the same website. BottomScanner allows searching and analysis of the auction contents for specific good buys. When it identifies something that is a "good buy" according to the rules you set, it will alert you and give you the choice to buy or bid on the item.
My second (and with my hunter) CT_Raid assist is a must if you like Dungeons. This one is a little complex to setup but will get you far more bars then you need, and allows you to move just about anything around. (This is only suggested if you like raids, and you have complex vision on your screen that needs to be changed, (like showing the right side bars, those could be moved/changed to your liking.) - This can be downloaded here CTMod - A World of Warcraft Mod ***WARNING*** I DO NOT RECOMMEND
CT_Unit Frames nor CT_View Ports Unless your an advanced player.
Recently Blizzard started making questing reward more exp so that players would enjoy the game more (or so i hear...) so to help people quest... the ultimate quest helper... here QuestHelper | World of Warcraft Addons | World of Warcraft @ Curse.com **Sometimes this website works sometimes it doesnt check it daily** This IS NOT CONSIDERED... a maphack, however all area's on the map are revealed, all quest giver's are marked and remembered, all quest objectives are shown, and they show you where to go... what to kill, howmany you need, and howmany's left... this is the ultimate quester...
*edit* This also show's enemy levels on map when hovering over a specific place, (like darkshire, or orggrimmar)
*Another Edit* i saw a post by moonlightparty that says "the arrow wont go away when i move direction" disable the arrow in the addon prefrences (i dont know exactly which one it is, but i know its off.... and aint comin back on...
now my final Addon recommendation is AtlasLoot Enhanced, and Atlasmap Whilest inside instances these addons allow you to see the map of the instance, and detailed at that... each boss, rare mobs, and (if your lucky) sometime in the future questhelper will be linked with this (in some parts it already is) Recommended for Swamp of Sorrows instance. But heres atlasLoot, its up to you to find atlas map... Atlasloot Enhanced | World of Warcraft Addons | World of Warcraft @ Curse.com
Enjoy- And remember if you add all of these addons expect your ping to raise by atleast 100 and if your on dialup expect your ping to raise 1000 but your lag remain relativly low... (you shouldnt drop)