None of these are required to rp, but are strongly recommended!
MogIt
MogIt is an in-game catalogue of transmogrification items with the aim of helping players build their own sets!
http://www.curse.com/addons/wow/mogit
MyRolePlay
MyRolePlay is a roleplaying UI AddOn for World of Warcraft, developed by Etarna at moonshyne.org. It lets you make a roleplaying “profile” about your character that other users of MyRolePlay (and other compatible “MSP” RP AddOns) can see in the tooltip and a more detailed window.
http://www.curse.com/addons/wow/my-role-play
Gryphonheart Items ***THIS IS STRONGLY RECOMMENDED, ESPECIALLY FOR OUR GUILD RP***
Create in-game items that interact with other GHI addon users. Write a book, a note, a love letter perhaps a poem, and keep it within your bag or trade it to other users of GHI. Create birthday presents, tools, food, drinks, weapons, banners – everything! Only your own imagination sets the bar.
http://www.curse.com/addons/wow/ghi
If you guys have any other recommended addons please post a link and I'll add them to the list.