Suggestion: Add-ons for guilds and players.

Instance Add-ons
Omen Threat Meter. KLH Threat Meter used to be the "big thing" of threat meters, but hasn't been updated in a while and can't track multiple targets. Use Omen instead. To download the correct file, look for the most recent update (by date) at the bottom of the page, excluding the xml file.
http://files.wowace.com/Omen/

CT Raid Assist, aka CTRA. You'll find it nestled among other CT add-ons at the following site, though only the Raid Assist add-on is needed.
http://www.ctmod.net/downloads/

Deadly Boss Mods, or its Ace2 counterpart BigWigs Boss Mods. The links are provided for both, though I like BigWigs better because it is lighter weight and I've never used DBM, but here you go.
http://wow.curse.com/downloads/details/4940/ (BWBM)
http://wow.curse.com/downloads/details/3299/ (DBM)

Guild/ Utility Add-ons
Group Calendar, for scheduling events.
http://wow.curse.com/downloads/details/1958/

Atlas, the instance map. It pwns.
http://wow.curse.com/downloads/details/354/

AtlasLoot, which gives the loot for all instances currently in-game. It needs Atlas to be the best it can be, and it pwns x2.
http://wow.curse.com/downloads/details/2983/

Cartographer, for being spiffy with a cool map add-on.
http://wow.curse.com/downloads/details/4947/