Package AddonsThese addons generally work best when used with other addons. Some can be standalone addons as well.
All Deuce UIA compilation of various Ace2 addons.
AtlasWith just the basic Atlas, you get valuable map information, such as flightpath maps, and maps of instances.
Atlasloot EnhancedWorks with Atlas, or as a standalone mod. Allows you to see loot information about bosses and minibosses in instances.
Deadly Boss ModsProvides tons of useful information about bosses in instances and raids.
FuBarThis addon is an interface for TONS of other addons which can be found on this same page. With the various "fu" addons, such as MoneyFu and HonorFu, you can get different helpful information, and display it in the FuBar. There are many addons that can be used with this one addon, with pretty much something for everyone to be found.
LightheadedA great addon for your quest log. This addon attaches a side-window to your in-game quest log. This side window contains all the comments and questions asked on www.wowhead.com for the particular quest you are looking at. This is helpful because people post their suggestions on how best to do a quest, and what to look out for. Also people post coordinates for where to go which is good to know if you have any coordinate system.
MazzleUIA compliation of addons designed to give you a pretty much complete UI overhaul without having to do and download and configure all the addons seperately.
Titan PanelTitan Panel adds additional information to your interface that some may find helpful. It can be used to show framerate/latency information, experience information, location information, how much money you have, how much bagspace you have used, and so on. Can also be used with various plugins
TomTomIf you get lightheaded, you should definitely get this addon. TomTom is a coordinate system that displays your coordinates on screen. The reason this addon is a must-have with lightheaded is that when someone posts coordinates in a wowhead comment, it shows as a link in lightheaded. If you have TomTom, clicking this link will set a waypoint on your map and minimap for this location, making it easier to get to where you are going quicker.