I use the Cosmos package of add-ons. Specifically, I've found that Character Viewer, Khaos (a configuration manager), MapNotes, MobInfo, and World Map Info have greatly improved WoW playability and my WoW experience in general.
Character Viewer allows you to view your other characters on that account/server, including their gear and the contents of their bags and bank. Also adds report pages that you can tab to when using the add-on that tells you how much cash each character has, how long (in levels) they will be rested, and where they are in the World of Warcraft.
Khaos adds a small button by your minimap that opens a menu to configure your Cosmos package.
MapNotes allows you to add tooltips and notes to your world and zone maps - handy for pinpointing things like specific NPCs, the inn in Darnassus, and where to go to complete quests.
MobInfo keeps track of a variety of statistics on mobs you've killed - including health, dps, the number you've killed, and what they've dropped. It displays this relevant info (you can specify what) with a mouse-over on a mob or a corpse and includes a search feature to look stuff up when you're not near the mob you're curious about.
World Map Info adds X,Y coordinates below your minimap and on your other maps. VERY handy if you use thottbot site for quest/item help as many people simply post these coordinates rather than describing where to find various things.
I also run the thottbot Cosmos add-on so I can contribute to that supremely helpful database. :)
Note that I've turned a fair number of the other add-ons in the Cosmos package off; some because they added too many annoying (to me) buttons around the minimap, others because they caused conflicts with the way I like to view the game (target of my target, enemy casting timer, etc). I think Archaeologist and ArcaneBar were the main offenders for this. Also, I turned the Durability add-on off, which adds a floating % above your character keeping you updated on your gear damage. Some may find it helpful but it just annoyed me for some reason.
I've also started customizing my User Interface to something that more closely fulfills my needs. The X-perl UI mod allows you to…well, do a number of things, including move your portrait, target portrait, and target-of-target portrait closer to the action so you don't have to keep glancing up at the top of the screen to check your or your enemies' health, mana, etc. It also allows you to arrange where on the screen your current party or raid parties are displayed. Furthermore, there's a lot of different toys included in this UI mod, such as animated portraits for you, your pet, and your target. This UI is very sophisticated and has literally hundreds of ways for you to customize your UI to your preferences.
Another UI mod I've been experimenting with is ArcHUD, which places your health, mana, energy, etc.bars as arcs around your character's sprite. Your current target's health, mana, etc. can also be diplayed around your sprite so that you never have to look away from the main viewing area to check how the fight is going. Colors, scale, and the degree of the arcs are all customizable and the mod also give you the ability to place a name plate that detail your and your target's vital data (including a 3D model of your target) and turn off the standard Blizzard nameplates (portraits) to clean up the top of your screen. A very nice-looking UI mod.
Those planning on participating in raiding should download and install Omen, a threat meter, so you can gauge how much hate/threat you are generating in comparison to the other members of the raid, especially the tank. Since threat meters can only compare the threat of people who have the addon installed, if you don't have it installed your threat won't register on the meter and nobody will know how much hate you're generating. This addon will give a warning when you get close to pulling aggro off the tank so you'll know when to back off a bit and is probably the most important addon you can get to help ensure the raid party doesn't wipe.
I really enjoy using the Auctioneer and Cartographer add-ons. Both have helped me make tons of Gold and find areas that I have stumbled accross and wanted to return at a future date. I highly recommend both of these add-ons to anyone that is interested in being competitive at the Auction House as well as in the Quests. Enjoy…
I use Titan Panel. It has coords, a money display, time display, the number of slots that are full and other useful features. Another useful addon I use is Atlas. Atlas has pictures of the world with all the flight paths, maps of all the dungeons and battle grounds.