Auctioneer installs into your auction-house interface and your tool-tips to give you up-to-date information on the economic situation on your server. Just go to the auction-house and set it scanning to populate the database. From that point on, every item's tooltip will have the price of the item when sold to a merchant, the average price of the item on the auction-house, and how many of the items were for sale the last time you scanned. A full-featured panel on the auctions screen allows you to find the best price to set for your items and to figure out bidding strategies, and the logs allow you to track your sales and keep track of both good people to buy from and good customers.
This little add-on has successfully turned me into an auction-house tycoon on every character I have played past level 30. The reason I say past level 30 is because the add-on isn't magic...it gives you an edge, but you still need to figure out the strategies yourself. If you have a head for business and a keen sense of markets, however, this add-on gives you all the data you need to get rich and have fun doing it.
As the name implies, Cartographer is a mapping utility. It installs into your normal map interface to give you coordinates, change the look and feel of your map, mark and color-code your group members and raid groups, and view maps that you normally can't view. It also allows for viewing battleground maps outside of the battlegrounds, perfect for planning your PvP strategies, and instance maps, especially indispensable with pick-up groups who never seem to quite know where they are going. The map keeps track of your professions, so you will never be at a loss for herbs or minerals again, and has robust note-taking features available on both world and instance maps.
One of the best features about Cartographer is the ability to mouse-over a zone on the map and show the levels, instances, levels for each instance, and the party requirements for each instance. It will even show the unexplored regions so you'll never run around lost again. No matter what you need to do or where you need to go, this add-on gives you each piece of information you need, just a key-press away.
Titan Panel is one of the more subtle and interesting add-ons in WoW, because it can itself have add-ons plugged into it. The add-on gives you a small horizontal toolbar across the top or bottom of your interface. On these bars are various pieces of information, such as your location, how much gold you have, your ping and frame-rate, and the current looting rules for your group. The size, font, and transparency of the bar is configurable, as is what information it shows.
What is so interesting about Titan Panel is the add-ons for it that give it greater functionality and keep track of other statistics, such as damage-per-second, warrior's stances, and buff durations. Because of this, almost any piece of information you can think of has an add-on for Titan Panel available, which makes this the add-on that keeps on adding on. You can even set both the top and bottom bars up for added room and fill both of them with useful functions and statistics.
Overall, Titan Panel is the kind of add-on you put in and then take for granted, only realizing how useful it was when you don't have it enabled. No install of WoW should be without it.
If you, like most players, get sick of the time-sinks involved in playing online games, this is the add-on for you. QuestHelper tracks the waypoints for each quest and lists them in the shortest possible order. This allows you to complete your quests with the least amount of travel possible, which for quest-addicts like myself saves an incredible amount of time.
This add-on really proves itself once you have your mount or travel-form if you play a druid, because you can just run from waypoint to waypoint without worrying about checking the maps or asking guards for directions to important NPCs. Even better is when you have warlock friends who can summon you when you find out you have a waypoint near them. Altogether, a serious quester's dream add-on that will save untold hours of trudging around Azeroth.
This add-on saves just about every high-level player I know save amazing amounts of time and effort in building up their characters. Atlasloot Enhanced allows you to pull up and browse the loot table of any boss in the game. It shows the stats, drop-rates, and allows the item to be "tried on" in the dressing room function.
This add-on allows players to plan their characters and go after their planned build so that they can maximize their character's potential, all from a single interface rather than hunting through webpages and fansites. Hardcore and casual players alike can benefit from being able to plan their gear this way, and this add-on makes it easy and fun, like going through a Christmas catalog and planning your list. A useful tool for any player who wants to get the most bang for their buck without the randomness factor.
A bit of warning: as of this date, none of these add-ons caused any problems on my install of WoW, Client Patch 2.4.3, released July 15, 2008. However, any add-on can cause problems in some configurations. Always install add-ons one at a time and login to make sure nothing breaks. That way, if you do have a problem you can uninstall the most recent add-on rather than having to try to figure out which one is the culprit.
Have fun and keep gaming!