Helpful Add-ons
Deadly Boss Mods: This addon gives you warnings on your screen about dungeon bosses and when they're about to cast something nasty. It also whispers people back to let them know when you're in a fight so that they don't assume you're ignoring them. Its CPU usage and size is small, so it's a nice real-time boss advisor you should check out. DBM is now split into several parts with each part pertaining to each expansion of the game.
AzCastBar: A customizable and lightweight casting bar replacement add-on. This allows you to move and change the look and size of your cast bars. So if you want your cast bar in a different place or just want to resize it for convenience of space or visibility, this is one you'll want to pick up.
GTFO: To put it simply, "Don't stand in the fire". Deadly Boss Mods already will notify you if you're standing in something that's dealing you a lot of damage, but if you feel like you'll need the extra loud alarms telling you that you really need to, well, "GTFO", then this one will be helpful to you. Your healers will appreciate it.
VuhDo: This is a healer's best friend. You can customize this to your liking so that you can easily see the amount of healing needing done on a party member, whether the party member is in range, if they have any debuffs that need dispelling, if they need a buff you're able to provide and so on. When you open up the menu and go to the spells tab, it allows you to place spells on specific clicks or hotkeys so that it'll cast that spell when you press the hotkey you specified -- For example, if you put a healing spell on your left click, and a debuff dispelling skill on your right click, then when you mouse over the VuhDo party list and left click a player it will automatically cast that heal, and when you right click them it will automatically get rid of their debuffs (cooldowns and such permitting of course).
Dominos or Bartender4: Both of these addons do the same thing - they allow you to add and move around and resize your action bars as well as other elements such as the vehicle bar, the stance bar, the bags bar, the menu, and so on. There really isn't much difference between the two, so it's a matter of preference.
Recount or Skada: DPS meters. These measure the amount of damage per second you and those in your party are doing. They can be helpful with seeing where you are, and if your dps is on the low side it can be a number of things from being lower level than those in your group, to having a problem in your rotation that could be tweaked. Which of these two you get is totally up to you - they both are able to report things like dps, damage done, healing done, etc., and both give you several options of where to report them in chat (guild, party, target). The main differences is that Skada is a little less CPU intensive and records the dps done until the end of the fight whether you die and have to walk back or are there at the boss the entire time, so it counts the time away from the boss where you are not doing damage and thus reports the effective dps done for that fight while in that group. Recount requires a little more work from your CPU, but has a simpler interface. Recount will record your dps while you're attacking a boss, so if you die and have to run back, it doesn't account for that time where you're not attacking so can sometimes be slightly less accurate than Skada. But overall, Recount is still a good add-on for dps watching purposes. Both of these add-ons can cause a little lag at times if you have a computer that doesn't or barely meets the minimum requirements to run the game, so if that's the case it's best if you keep them disabled until you really need them in the higher levels.
Auctioneer or Auctionator: Both of these addons are helpful for working the Auction House. Auctioneer is more complicated but more in-depth and will notify you when someone has undercut you. Auctionator has a slightly simpler and easy to understand design for those new to the idea but will not notify you of undercuts unless you manually check for them yourself. Both will do the math for you and show you what items are going for on a per-item basis so that you can get the most profit for your items - checking the standard auction house can be tedious and can result in severe undercutting and loss of some profits if you don't do the math. Both these addons let you scan the auction house so that when you find new items and want to know their current market value, you need only mouse over the item to show the tooltip and it will give you the auction value per item on the tooltip right there so that you don't need to run and check the AH or ask someone to check it for you as long as you keep it scanned regularly. Both good add-ons if you want to try to make gold off the AH.
Postal: This addon will allow you to open all your mail at once without having to individually click each mail to get it out of your mailbox. It gives you the option to choose how many empty slots you want left in your bags while opening mails, and makes bulk mailing to alts, guildies or friends much faster. This is also a great companion to one of the Auction House add-ons listed above since it'll gather your profits quickly and tell you the grand total at the end in your chat frame, as well as allow you to more quickly relist items that have failed or that you have cancelled because someone undercut you.