Here's a few that I use:
SmartBuff: Preset what buffs you want for solo, party, raid, battleground, dungeon play. If anyone needs buffed, it tells you. Spin your mousewheel to cast the buff.
SmartDebuff: Makes a little frame with everyone's class color and name. If they get a buff that you can remove, it turns to (blue background, L in the center) or (red background, R in the center) meaning you need to L- or R-click that box to clear the debuff. You don't have to target the person and remember which cleaning spell to cast. You don't lose your focus when you click on this box.
Grid: Similar to the SmartDebuff box, this creates a little set of boxes for everyone in the party. It's much smaller than the default unit frames, and gives you basic information about how many health points the player is missing, how much your heal is going to heal them, if they're buffed/debuffed by things like Renew or PW:Shield. It can also have a visual indicator if someone is debuffed by something you can cleanse.
Clique: let's you set key-combinations for any spell. For example, if I shift-left-click any player frame, they will get a renew. If I Ctrl-left-click any frame, they get a flash heal. Ctrl-Right-Click gives them a shield. It works on default party bars, UI bars, floaty (press v) bars, grid bars...pretty much anything.
Deadly Boss Mods: tells you information about what bosses are doing that you have to watch out for. If there's a special bleed on the tank, you'll know to start overhealing, if the boss is spitting fire at a specific player, they'll know to move, etc. Hard to explain, but awesome when you see it working.