Addons are great at saving us time helping us focus on boxing our toons.
One of MiRai's threads has influenced me to do alot of addon research. Here is a list of some of the addons that I prefer to use. Its not a complete list. I'm not active but I have been writing down on scrap paper and thinking of what addons I would use if i resubbed.
I listed the name of the addon. A short description and a link to where you can download the latest version. A picture is worth a thousand words. A small number of them have YouTube videos as well.
Most of us dread having to redo your addons because its so tedious and complicated. help is out there. Luxlunae has a great writeup on Addon Management, and he has a script to copy settings from one account to another. MiRai shows us his trick to configure Multiple UI's here. It uses mouse repeater to do this so understanding resolution and cursor sync is critical.
Convenience
Jamba - big suite, my favorate: jambastrobe to help prevent follow breaks, esp in air or water YouTube
GTFO - warns you if your steppin in fire, its possible to get a second or two advanced warning. To hear slaves audio check off "sound in the backround" located in Options/Sounds . YouTube
Range Display - digital range display. Stop at 20 range from your target and the slaves following 10 range behind you will be in most range envelopes. stop, assist, nuke, follow. rinse wash & repeat as desired. YouTube
Smart Buff - semi auto buff casting. I attach this macro to my follow key and never have to worry about buffage. YouTube
Cant Heal You - healer sends tell to target that is too far away to heal
Camera Plus - GUI helps unlock default camera max zoom out limit without slash commands YouTube
TrufiGCD - See visually outgoing casts next to player frames. uses: troubleshooting and DPS tweaking Youtube
CinematicCanceler - Blows past cut scenes you've allready seen
Inventory Management
hotloot - streamline the looting process YouTube
hotloot notify - alert and notify the user when specific items are looted.
hotloot history - track items you loot and you can search through and filter them.
Combuctor - one bag, filterable using tabs YouTube
combuctor mining - filters bag to show only stones, ores, bars etc..
combuctor tailoring - shows only tailoring based Materials, Crafted Armor, Bags, Recipes
combuctor scrap - adds scrap catagory bag filter, scrap icons glow
combuctor skinning - adds skinning bag filter to show leather mats, products and tools.
combuctor jewelcrafting - addons jewelcrafting filter to bags showing jewelcrafting mats, products and tools.
combuctor glyph - simple glyph tab
combuctor bindtwoaccount - allowing easy look up of items that bind to your Battle.net account
Genie - automatic bags sorter YouTube
PassLoot - auto loot rolling based on advanced set of variables YouTube
oMail - helps offload bags. mails off items to designated toons. cloth to tailoring alts, ore to blacksmithing etc...
Progression & Leveling
Pawn - calculates scores for items to help you easily find upgrades for your gear
Pawn Auction - helps you find better gear in the auction house (Hpac had a post looking for this)
autoprofit - sell greys and gear you cant wear
xp2level - provides details about your leveling progress
tabardmaster - switches gear based on role
lose control - lets you know if a party member is feared
Team Equip Compare - helps decide with gear drops shows groups gear for slot
Performance Enhancers
weak auras - buff, debuff, cooldown tracker supports group tracking YouTube 1 2 3 4
PvP
healershavetodie - adds a red cross over enemy healer's name plates, useful in mob packs in PvE to find the healers and burn them down YouTube
Professions & Farming
NPCscan - tracks seldom-seen rare mobs by proximity alone, making the old “/targetexact …” macros obsolete
NPCScan overlay - adds mob pathing/territory info to map YouTube
NPCScan autoadd - adds features to NPCscan YouTube