There are a couple of essential addons that you are required to have installed if you wish to raid.
These addons assist you with boss encounters and also help the raid leaders as they present us with lots of essential information.
Firstly, you need to have installed a boss encounter addon. The two you can choose from are Deadly Boss Mods (http://www.deadlybossmods.com/) and BigWigs (http://wow.curse.com/downloads/wow-addons/details/big-wigs.aspx). You only need one of these installed, so pick your preferred one. Deadly Boss Mods (http://www.deadlybossmods.com/) is by far the most popular.
Next you will need oRA2 (http://wow.curse.com/downloads/wow-addons/details/ora2.aspx). This will enable the raid leaders to see lots of info about your current status such as cooldowns.
Omen (http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx) is also required to keep an eye on the amount of threat you are generating so you don't pull aggro off the tank.
So, to recap. Please install the following as a bare minimum.
Deadly Boss Mods (http://www.deadlybossmods.com/) OR BigWigs (http://wow.curse.com/downloads/wow-addons/details/big-wigs.aspx)
oRA2 (http://wow.curse.com/downloads/wow-addons/details/ora2.aspx)
Omen (http://wow.curse.com/downloads/wow-addons/details/omen-threat-meter.aspx)
Class Specific Addons
Paladin
Pally Power (http://www.wowinterface.com/downloads/info6370-PallyPower.html) is an add-on that provides an interactive and easy to use interface that allows you to control assignments of blessings for yourself and, if you are a party leader or an assistant, for other paladins in your group or raid.
NOTE: This is a work in progress and this thread will be updated as an when new addons are required. If you have an addon that you feel is essential to your class, please PM me with the details.