WhisperCache

WhisperCache is a whisper caching solution that will allow you to never miss a whisper again. Each time a player whispers you, the message is saved across all characters until you reply to the person or manually view your missed whispers.

Simply type /wc in game to view your missed whispers at any given time.

Download

You can download WhisperCache on the Minion/Twitch app or through any of the following locations:

Curse: https://www.curseforge.com/wow/addons/whispercache
WoW Interface: http://wowinterface.com/downloads/info23775-WhisperCache.html

A bleeding edge version of WhisperCache can also be used by visiting its GitHub repository here: https://github.com/snail23/WhisperCache (The files here can change many times a day so it's not recommended to use this version of WhisperCache unless you know what you are doing)