The 2 core addons I use are Titan panel and Nurfed Mod. Below are the links to the official pages. They are really nice and not that patch sensitive as many other UI addons like NP-UI or Cosmos-UI. Though this is the mods I use and like. It is only a small peak on what I use ingame And that it hopefully might help someone out a bit.
Remember to read all the pages the addon are located on. There is much usefull information there.
Titan Panel's official developer page
Download page for Nurfed.
The rest of the mods i have downloaded from http://www.diabloii.net/
But I have linked to all the pages for the individual mods/tools below.
K-WOW - Multi Tool
Is a really good multi tool for WoW. I use it alot.
Atlas for instances
Here is the cure to not get lost in the Instances. Shows all infor you need really good.
Auctioneer
Want a nice helpfull tool to make you more successfull on the AH.
Equip Compare is attached to Auctioneer. Really nice to have.
FlightPath
Nice tool that tells flighttime and cost. Visually marks the flightpath on maps.
LootLink-1.6.1
LootLink Backup and Restore
Really nice database that stores item info for you to search through.
BankItems
Really usefull tool. this tool I really love. No more loging out to another char to browse the bank. This does the trick.
Stancesets
Changes your weapon types after stances. Easy to configure and really handy. Dont forget to download WeaponQuickSwap ADDON that is linked on the page.
Goodinspect
Tired to lose the inspect window all the time. This extends the inspect range so you can look as long as you need to.
MailLog
This makes a logfile of all sent mail ingame with or without attachments.
Decursive a nifty addon for Paladins and Mages.
Regards from Felakin