Thanks for the explanation of addons. Some people can be real purists about that sort of thing and see addons as cheating, but that is just silly. A good deal of the current WoW interface exists because of popular addons that blizzard have taken onboard and added to in game functionality and more is coming in WOTLK.
Curse is a good place to go to get addons. The important thing to look for when deciding which addon to use is the rating, user comments, and last version update. It is always a good idea to go for something that is well liked and updated on a regular basis by the developers. The key is to check back regularly to see if any of the addons have been updated.
On that note, once you start running addons, I highly recommend another program called Wowmatrix. It is a stand alone program that you download off the web for semi-automatically updating your addons. Just download it onto your main drive -- it works fab. It scans your addons folder and then tells you if any of your addons need updating with a newer version. You can either update each program individually or use the update all option. It does all the work for you in a few minutes time as opposed to constantly going back to curse to check. I just pop it on every day and do a quick one or two minute update -- yes, the programs can be changed that often. You can also add new addons with wowmatrix or uninstall ones you have, all from the same interface.
My can't live without addons: auctioneer -- one time my auctioneer stopped working because of a delay from the developers in updating it for the latest WoW version and I just stopped using the AH until it was fixed. It offers so much functionality and takes a lot of the guess work out of auctioning off your stuff and tells you if what you are buying is at a good price or not.
some sort of map program -- I use cartographer. I used to use atlas with other map bolt ons like mozzfullworld, but cartographer has it all. It will even remember where you have previously gathered mats from before (ore, flowers, treasure chests) and mark them on your mini map. It makes gathering a breeze.
I don't use them, but if you are new to the game or have a poor memory, then get some sort of quest helper program. I have questhelper loaded myself even though I rarely use it. It seems to be the one most of the people I know use. It suggests order of doing quests based on efficiency and marks on your map where quest can be found, the area that quest objectives are in, and where to hand back in. It even suggests the best path to follow. It also adds a line to the mob descriptor box that appears on the bottom right of your screen when you put your cursor over a mob. It will tell you if that mob drops any of the quest items you are looking for or whether or not killing it will fulfill a quest objective.
My addons you could probably live without, but are nice to have:Spammenot has to top this list. Never look at those horrid gold selling messages in your chat window again! It is one of the best spam filtering programs I have used.
Wim -- this is a program that creates an MSN style dialog box on screen to handle whispers rather than putting them in your chat window. Have you ever sent a 'whisper' out in guild chat that was somewhat embarrasing? Then this program is for you. It also helps with sending the right whisper to the right person if you are in multiple whisper conversations. It won't pop up if you are in combat, so the window is not obtrusive (if you are in combat when someone whispers you, it goes into the regular chat window as normal). So the next time you say 'f u' to a friend who is being cheeky in whisper, you won't end up with egg on your face because you accidently said it in guild. lol
Threatmeter -- a must have for end game instancing. I recommend Omen for this as it takes into account your pet so it is very useful for locks and hunters at any level of the game. Another reason I like this over KTM is that it gives you an audible and visual signal when you surpass 95% of the main aggro target's threat. A great wake up call that says slow down on the dps or you are going to pull aggro off the tank and then you will be despised and loathed by your fellow party/raid members.
Other popular addons:Damagemeter -- not particularly helpful, but a lot of people like to run them. They are particularly popular among male players who like to flash up the dps stats at every opportunity to show they are top of the heap. We won't go into the freudian aspects of why that is. They are very useful for raid leaders as they can give an good overview of raid member performance (raid leaders look at more than just dps and being the top of the dps board is not always a good thing).
Bag/Inventory addons can be useful for keeping your things organised. There are some that also allow you to see what other characters have (on the same realm).
Trade skill addons -- these can help by allowing you to queue up your crafting or tell you if you are missing any usable recipes from you skills, or telling you if you have the mats you need for a craft in your bank or on another character.
Buff/debuff programs -- tell you when you or a party member are missing a buff or have had a debuff put on you by a mob.
CC counters -- puts a mesage in party/raid chat that a mob has been cc'd and gives a countdown as to when the cc will end. It will also tell you when cc has been broken. Again, very useful for instancing and group quests.
I run various other programs for class specific characters, but those are a personal choice and really depends on whether you find them helpful. You need to remember that all these programs take up memory, so the more you run, the more impact it will have on your pc performance while on WoW. You don't need to run all of these for all of your charcters though and they can be turned off individually so each character is running only what they need or use.