Here's a short list of add-ons I'd recommend:

1. KLMThreat - this is a threat meter. It lets you keep track of how much threat you are generating, as well as any other party member that has it installed, too. This is especially important for tanks and clothies (who don't like getting hit.)

2. GroupCalendar - this add-on give you a calendar to schedule events, like raids, dungeon instances or even mundane things - what ever you want. It has guild integration so guildies can share events. Very handy.

3. Bagnos - tired of opening each bag looking for something. Bagnos lets you use all your bag slots in one window.