Crusader Kings II
They're using a newer updated version of it for EUIV. They modified the engine for CKII because people petitioned for a CKII save exporter to import into EUIV. So they made sure the provinces and country tags and all that other stuff exports properly and matches up with the ones in EUIV.
Otherwise we have these player made exporters which when imported put Russia on the Spanish peninsula and Spain as a one state province in the HRE. :P
The Customizer was also a big request so people can rename their Kingdoms and Duchies without having to mod.
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem