My Roleplay is a roleplay profile addon that provides basic functionality but may not see everything transmitted by other addons (such as custom character class).
Total RP 3 is a feature heavy RP addon that drastically changes your tooltip. Not all features send information through the universal protocol and thus not all features are compatible with other addons.
XRP is a moderately feature heavy RP addon with as much compatibility as possible with other addons with a few features unique to itself (profile across battle.net and across server when in a group with the other player) and is a middle ground between MRP and TRP3.
Other RP Addons
Gryphonheart Items allows players to create items using its own scripting features and lua. Please be sure to configure it to not allow scripts you run to make you do things without your permission, as trolls sometimes use this to bother RPers.
Other Addons Useful for RP
Squelch is an addon which mimics the in game report spam feature without the reporting. It temporarily hides text from a player and removes their previous text from your chat window. This is good for filtering out long posts in busy areas.
Listener is a chat addon that helps you keep track of messages and sends audible cues when people emote at you or people you have targeted talk.
WIM gives you in game messenger windows for whispers. This can be helpful for guild leaders and others who are afraid of missing whispers.
Global Ignore List extends your ignore list and copies your ignores across all characters on a single wow license. Sometimes, though, it loses all of your ignores so you want to keep a backup of the Global Ignore List saved variables file just in case.