There are several accepted programs that help add to your World of Warcraft experience without violating any rules or laws. These programs consist of chat programs, meters, and slight interface changes. Some of the chat programs like Vent are basically required for any type of major raiding. The meter progams allow you to gauge the healing or damage of your group while interface programs allow you to get prepared for battle. 

CT RaidAssist

Cosmos UI 
Application that gives you various tools to improve your World of WarCraft experience. Please note that this is not a cheat. This app will track your stats while you play so you can analyze them in order to constantly improve.

Ventrillo 
The number one tool for end game instance raids such as Molten Core, AQ 20, and AQ 40.

Skype 
A leading peer-to peer Voice Over IP program that allows small groups of players to chat for free. Fun for small guilds who do not do much raiding.

CT Mods 
Various mods such as CT RaidAssist (screenshot above) that allow you to do many things such as monitoring the health, rage, energy, and mana of your raid.

Team Speak
Another chat program thats allows for chatting while in a group or raid, but based on the client-server model instead of peer-to-peer.