One of the cool things about World of Warcraft is that they provide an engine that will parse UI Addon scripts which are designed to make your gameplaying easier and more convenient.
What do you mean, "More convenient"?
Ah, so glad you asked ;)
Basically, the scripts help automate things. My scripts do things like:
Feed my pet when his comfort goes below "happy"
Automatically build a database of what my different pets like and do not like to eat, on the fly (ie: as my pet is trying to eat, it updates and retains the information for next feeding time)
Automatically switch combat bars depending on whether an opponent is in a Range space or a Melee one compared to me.
Make creating and maintaining raid groups easier in the battlegrounds like Warsong Gulch, etc.
Building a database of fish I've caught in different areas
Automatically re-casting after catching a fish or after one gets away
Give extra information about objects and NPCs when I hover over them with my mouse (auction prices, disenchant information, whether animals are tamable and what skill they'd give hunters if tamed, etc)
Scripts?? I can't make scripts!!
Me either!
Thankfully, there are many websites available that offer the UI AddOns that other people have created, which means you don't even have to worry about learning the scripting language to make things work better!
Download notes
Of course, as was mentioned in my previous entry, always be careful when you download things online. Some sites don't review the content of what is uploaded, and you may end up with a virus or trojan installed. Always make sure you do regular virus and spybot/trojan scans.
Popular sites with free WoW UI Addons
There are some sites that offer membership access to their scripts - I personally move away from those sites very quickly, because I'm not looking for scripts of such complexity that I feel I should need to pay a membership fee. You can make that decision on your own.
Here are the two main sites I've used myself for improving my WoW User Interface for my Hunter character:
http://www.curse-gaming.com/mod.php
http://ui.worldofwar.net/
Bundled or individual?
One of the first UI mods I used for World of Warcraft was the Cosmos UI AddOn, which is a bundle of useful mods put together which also gathers information for Thottbot. However, my computer can't handle a lot of active UI addons, so these days I've been downloading non-bundled scripts.
Things to consider
While Addons/mods are wonderful things, you need to consider how well your computer performs before loading your game down with them.
My computer has 512MB RAM and 64MB RAM on my video card, so it's in the "low range" of being able to handle many mods. I initially had the full Cosmos UI package installed but I found my game was painfully slow as a result (as soon as I removed the addons everything was nice and quick again)
UI Addons and WoW patch updates
One last thing to note is that your AddOns are disabled every time there is a WoW patch update, which may be every week. You can re-activate your addons and even make your addon list disregard whether a script is outdated or not when it comes time to load.
But I'll talk about those details in a future entry!