Raid AddOns generally improve Raid performance. There are various mods which offer features such as main tank lists, raid warnings (messages which appear mid-screen for all raid members), voting mechanisms and boss status trackers. Another useful utility for raids are voice communication programs like Ventrilo or Teamspeak. Install them even if you have no mic - unless you're the raid leader it's not imperative that everybody can hear you.
The first (and for about one year the only) raid addon was CT_RaidAssist (CTRA for short). Meanwhile Blizzard added some of its features to the standard UI, but it is still the reference. It offers a nice overview for 25 or 40 raid members, raid messages, a main tank list and a lot more. The CT team still supports this mod, and keeps enhancing its functionality. It can be found at CT Downloads.
Meanwhile, the Ace community grew up to the point where they offer a mod for about every need. Notable Ace mods for raids include oRA2 (CTRA replacement), BigWigs (boss mod), Grid (very minimalistic but powerful raid group display) and many more. All Ace mods can be found at the WoWAce download site.
Aggro monitors are also very useful tools in raids. It helps a lot if the main damage dealers know exactly how many percent of the tanks aggro they already have reached. Notable mods for this function are Omen (Ace2) and KLH Threat Meter.
Boss mods keep track of the special abilities of most raid bosses, and inform the raid when certain critial events take place. The first of this kind was CT boss mod (found at the same place as CTRA), the Ace community offers BigWigs, and Deadly Boss Mods also serves this purpose.