Possibly one of the longest-running OpenXcom code mods is Soldier Diaries / Commendations, which keeps detailed stats of all your missions, allowing you to revisit your successes and failures anytime you want. Mostly failures.
However, being a separate branch meant it always trailed behind the official version, leading to many old bugs, compatibility problems and installation difficulties keeping players from trying it out. But no longer! Soldier Diaries has now been integrated into the official version, so now everyone can crack open the spreadsheets and write fanfics about their soldiers.