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.

Add-ons are a great way to enhance your gaming experience. Curse knows this and wants to help. Go to http://my.curse.com and register an account. Then go to http://www.curse.com/client/ and download the Curse client. Run the client so it can figure things out. If you have add-ons already installed, it will keep them up to date (which is really important because many of them break when Blizzard releases patches.)