oRA2 is a requirement for many endgame raid groups and alliances, including The Honourbound Alliance. It's CT_Raid-Assist compatible and provides additional raid frames and information. Some of this has been replicated in-game now, but it's still required, even if you don't use it yourself, because of the additional support it provides healers, tanks, and raid leaders.
Note: This mod is updated frequently. However, unless someone tells you there's important bugs that have been fixed, you only need to update this when a new patch comes out.
Usage
It's downloaded and installed similar to any other mod. Unzip the archive, open up the archive folder, and place the oRA2 folder into your Interface/AddOns folder and restart World of Warcraft.
It adds an icon around your minimap. Right-clicking the icon gives you some options. Most of you will not need to tinker with the options. Drag-clicking on the icon will allow you to move it.
If you're a healer, you can set keyboard bindings in WoW's keyboard editing preference pane that will map the main tanks, as set by the raid leader. By left-clicking on the Ora icon and choosing a slot, you can also add the targetted player to a set of player targets that can also be viewed on your screen or assigned to hot keys (again via the WoW keyboard shortcut editing preference pane).
Raid frames it shows itself can be moved on the screen by holding down the Alt (or Option) key and dragging.