Karthis at Of Teeth and Claws has a nice post up about the four types of addons that every raider needs, with examples of each. I thought I'd share which mod I use for each of his categories, plus one bonus addon that I consider essential for casters.
Encounter Adviser - the mod that tells you when a boss will do what
I'm currently using Deadly Boss Mods on my mage, but some recent comparisons with my fellow guildies have me almost convinced to switch back to BigWigs.
Enhanced Raid Frames - the mod that shows the status of your fellow raiders
Since I'm a dps caster, I really like the condensed view of the raid that Grid provides - in the addon world, it's the raid frame mod with phenomenal cosmic power... and itty bitty little living space.
Statistics Gathering - the mod that lets you review your leet skillz after the fight
My raid leader uses WoWWebStats, which means I don't have to install an addon like Recount. More FPS for me!
Threat Meter - the mod that lets you - OMG I HAS AGGRO! *invis*
I use Omen for this, and highly recommend it over KLH Threatmeter.
Bonus Category for Casters:
Modular Cast Bar - the mod that helps you reduce the effects of casting delay caused by lag
So far, Quartz is the only worthwhile mod in this category. Quartz provides a new cast bar that will show you how much of your cast is longer than necessary due to latency issues, allowing you to shave off some of this time with /stopcasting macros. Here's an example of a /stopcasting macro for fireball: