Hey everyone! Feel free to append to this list as they see fit. These are just a few really good RP addons that you can download to enhance your in-character experience.
Total RP 3: While MRP and XRP are fine choices, TRP3 is an insanely deep character profile generator. It even allows you to choose theme music, and changes your chat to display full RP names with custom colors. There are tons of great features on this addon, so feel free to give it an install and play around with it. I really prefer this to MRP/XRP.
Gryphonheart Items: This is an extremely fun addon. With this addon, you can create custom items with a myriad of effects. They have their own bag(s) that don't clutter your inventory, and even allow you to trade items between other GHI users. (Plus Bathildis will love you forever if she can finally hand out all of her brew-samples!)
Mogit!: Mogit is a dressing room app that allows you to create custom transmog set wishlists, and save/export them. It's nifty when you're looking for a new look, but don't want to start the grind without previewing it on your character first.
UnlimitedChatMessage: This is a chat addon that allows you to continue typing past the maximum allotment of characters in WoW. Once you have surpassed that maximum number, it just starts a new line of chat. When you finally hit enter, the entire wall of text is displayed as multiple chat entries. It saves you from having to worry about how long a description or dialogue is.
Misspelled: An in game spell-check. Not sure if it'll make suggestions, but it will highlight misspelled words, making it easier to catch your mistakes before you make them.