Edit: Ive taken the time to put the memory usage of each add-on that I have currently installed next to the add-ons so that you can know what to expect.
**Mandatory Raiding Add-ons**
Not going to spend time explaining what these do, as regardless they are necessary.
**Recommended Raiding Add-ons**
These, however, are for those who LOVE add-ons, and will get a full description.
KUINamePlates - (.1 mb) Changes the way the default Blizzard name plates look for enemies and friends. Tends to be a little neater as far as stacking/visuals. Also very customize-able as far as what information is actually displayed on the bar.
TellMeWhen - "TellMeWhen provides visual, auditory, and textual notifications about cooldowns, buffs, and pretty much every other element of combat"
CooldownCount - Shows a count-down on abilities to tell you how long until they are up. (Very similar to TellMeWhen, but the counters are on top of the buttons, not in a seperate section, and is limited to that one function)
WeakAuras - (3 mb) I can't even begin to explain how amazing this add-on is. Basically can show you the current time remaining for DoTs on the boss, the number of Starsurges you have left, whether or not a CD is up, and has even been modified heavily by Feral Druids to show the current power of their DoTs compared to one they could apply now to prevent applying a weaker DoT. Seriously. #WorthIt.
Requires a decent amount of setup, but often you can find import scripts for it.
Skada - (1 mb - 15 mb) A DPS meter that can tell you a whole lot. This can list when and how you died, your total damage, your DPS, healing, what spells did what % of your damage, along with what % of another person's damage spells did. This addon is really as useful as you make it, if you spend all raid staring at the meter and not the ground under you then I wouldn't get it. However, if you are a huge math/numbers person and you like to compare numbers/check what killed you etc then you can get a lot of value out of this.
Bartender - (2.8 mb) Allows you to move the default blizzard spell bars around your screen, and change them to have more/less buttons available, and change keybindings on a per-character basis.
Parrot - (2.1 mb) A Combat-Text add-on that can make viewing damage or healing easier as it is able to seperate the two on both sides of the screen. Can also be used as a cooldown notifier.(Currently returning a 404, but im sure is find-able somewhere else)
Quartz - (.7 mb) Custom cast bars. You can move your cast bar/enemy cast bar/focus cast bar etc to wherever. Also can allow you to display whether or not a spell is interrupt-able, along with time left till cast is complete/total cast time, and modify the size of the bars.
Shadowed Unit Frames (6.7 mb)/HealBot - Alternate raid frames. Highly recommended for everyone as it can make your UI ALOT cleaner than using the default Blizzard ones. HealBot will allow you to setup different mouseover casts, and probably show buffs if setup correctly. I prefer Shadowed Unit Frames for my self,enemy, and focus frames, purely because of the look of the frames.