I built an integration for a charity that processed many millions per year. The money went to support needy folks in an impoverished nation. Children could get

I built an integration for a charity that processed many millions per year. The money went to support needy folks in an impoverished nation. Children could get sponsors for schooling, food, and orphanages. The high-impact part comes from the organization and their mission moreso than my contribution to it, but it was also the most technically challenging work I did (shoehorning previous functionality into places it didn't belong and all the fun that comes with that). It's been about 9 years and I can see that largely, my backend is still running. The site had a facelift since then, though. This was a fully custom project, with a pretty standard Rails backend. The complexity was mostly dealing with Convio, the CRM / payment processing system from Salesforce for nonprofits.