[dragonshed]

For me, by far the most impactful is: The logistics planning software used by a major sports league in the US. The package I worked on allowed management to comb through a full season schedule, fine-tune a myriad of different associated weights, and push it through a specialized simulated annealing program to yield the schedule for every player, referee, etc for the season. The league went from manually producing perhaps 2 to 3 full schedules a season, then having to almost immediately scramble to handle exceptions, to producing hundreds.

Twenty years between various agencies and contract terms, I've worked countless projects, and the ones which improved business processes, where end users lives were made much easier, were the most rewarding. Conversely, most of the 'fun' projects were almost immediately obviated (I'm looking at you, Silverlight).