This addon feeds off the "Spell Alerts" that Blizzard added to the game in Cataclysm, incorporating many of the various HUD-type spell/proc alerting mods out there. The downside of the builtin Spell Alerts is that for new players, there's no easy way to tell which flashy thing in the middle of the screen corresponds to which flashy button on their action bars, especially when their talents are proccing often in a short period.
What Just Procced is a training helper for the builtin alerts. When a spell alert is shown, it will print the name (and link if possible) that procced the alert, along with where in the HUD the alert is displayed.
Use /wjp to toggle the addon on and off; it will always start turned off.
If you're a reasonably proficient speaker of non-English languages, any translations you can do would be welcome! Just make changes at http://www.wowace.com/addons/what-just-procced/localization/ and then send me a PM to notify me. I can offer nothing in return except public credit, my appreciation, and a beverage of your choice if you're ever in town.