Omen (threat meter), ClassTimers (DoT tracking as well as trinkets, bloodlust, meta gem procs, etc), Grid (raidframes), Quartz casting bar with /stopcasting macros (damn near required if you want to maximize your damage - allows you to "cancel" your spells early and still have them go off). DeadlyBossMods really helps in raids, especially in 25-mans.
I'd also highly recommend some kind of bar mod (I use TrinityBars personally), new unit frames (I use Perl Classic), and some kind of scrolling combat text (just getting into messing around with a few different ones now - currently using SCT/SCTD). If you're looking for a damage meter, I'd highly recommend Recount; as a raid leader, that's easily the #1 tool for figuring out who's pulling their weight in a raid and who's slacking off or not doing things right.