The two most popular addons for raids are CT Raid and oRA2.
I personally like oRA2 between the two, because you can configure a lot of what you will share with the raid and its entirely compatible.

As far as warnings, Deadly Boss Mods and Big Wigs are very popular for raids.
I actually raid with both, mixing and matching the features.
The BigWig warnings are stronger, but I like the whisper blocking feature on DBM.
Mixing and matching these two, by choosing which feature is enabled or disabled on each, is a strong combinaiton.