Important WoW Raiding Addons

The main aim at level 80 is to clear the end game content. There are many addons out there that will assist you in doing this and will make it easier to assess your own performance and the performance of others.

Deadly Boss Mods
Having an addon like this is key to making sure you don't fail. Deadly Boss Mods gives you important information during boss fights like when the next phase begins or when adds are spawning or when the boss is going to use a special ability. The advantage of having this is that you can prepare for the next step like on Anub you could get to the centre ring a bit quicker or on Sapphiron you could move away from the boss earlier to make it easier to get behind an ice block. It also records how long your fights have taken and you can compare how your group has done with previous weeks. On top of all this it isn't very memory consuming.

http://www.wowinterface.com/downloads/info8814-DeadlyBossMods.htm

Omen
Omen is a lightweight threat meter. Having a threat meter is important because as a tank you need to make sure that you are putting out enough threat to be ahead of everyone else and as a DPS you need to make sure that you are below a tank. There are other threat meters but I personally use Omen and it is very customisable and never had a problem with it so far.

http://www.wowinterface.com/downloads/info8459-OmenThreatMeter.html

Recount
Recount is an addon to track statistics, such as DPS and TPS, it also can track how much healing has been done and many other statistics. It is good to compare yourself to other players and you can report your results in party, guild, whisper etc. In a pug you can see who is straggling behind and you can use it to compare how much damage you do in different gear against a dummy.

http://wow.curse.com/downloads/wow-addons/details/recount.aspx

Simple Marker
This is only really for tanks or raid leaders to use. It makes it much faster to mark targets so there should be less mistakes with dps pulling aggro off a target with low threat.

http://www.wowinterface.com/downloads/info12131-SimpleMarker.html

FailBot
This one is also only recommended for tanks or raid leaders. It doesn't really do much in terms of help but it reports in raid channel if anyway has done a 'fail' some of these include, the slimes after Patchwerk, the Thaddius jump and messing up the charges on Thaddius. It allows you to see who is slacking and it may embarass them and may want them to rectify their mistakes.

http://www.wowinterface.com/downloads/info11819-FailBot.html

Grid
Grid allows you to see the whole raid in nice neat format very nice for healers as they can click a member on the grid and heal them that way. Very customisable and will save space rather than party frames. Also shows debuffs etc.

http://www.wowinterface.com/downloads/info5747-Grid.html

These addons are not required by any means and it may be more challenging without them. Also there are other addons which do basically the same job, but I like these ones.