AGCEEP is most popular modified version of EU2. There is a website for it, which has this introductory text:
Soon after the release of Europa Universalis II a group of enthusiasts started the "Event Exchange Project". The purpose of the EEP was to provide players with additional historic events. It also provided what-if events for countries usually doomed to obliteration, like the Byzantine Empire.
Eventually a great schism arose within the EEP causing the "Alternative Grand Campaign" Project to split off from the EEP. The AGC Project sought to also include more historical events and focused on providing more options to allow players to play the version of the Grand Campaign that they wanted to. Eventually the EEP and the AGC were able to put aside their differences and recombine their efforts into the AGCEEP Project.
The AGCEEP has created a large set of new events, modified many existing events, and added or modified leaders, monarchs, graphics, AI files and scenario setup. The AGCEEP has also taken on other mods as part of it such as Alun's Reformation Mod and Peter Ebbesen's Manpower Mod as well as the AI files of Daywalker's greatest AI mod.
Paradox forum: AGC-EEP historical mod