Two projects come to mind. Both were reporting tools. They helped companies improve the efficiency of a process 100x - 1000x. This resulted in the process being run more often and delivered multitudes more value in the form of better communication and knowledge transfer. They were fun to build and delivered tremendous value.
The first was at a fastener wholesaler. There was a sales report that would be built once a month by a sales person. It pulled data from a backend ERP system, combined it with some other data sources, and generated some trend graphs and summaries. The whole process took about five days of manual labor. It used to be done once a month, by a sales person. I've built a set of tools to do this which would take an operator about 10 minutes to run. It ended up being run daily, weekly, and quarterly. Everyone was aligned and on the same page!
The second was at a financial firm back office. One of the side responsibilities was generating reports for clients. These contained mundane things like trades summaries of various kinds. They were delivered in CSV or plain text format via FTP or email. It used to take a couple hours of work to build a new one, and output options were limited. I've built a reporting framework that added a bunch of output options, including beautifully formatted Excel files, and cut the config time down to about 10 minutes. By the time I left, we had over a thousand such reports being run daily.
Both projects opened up my eyes to how much value good software can bring to an organization, and how much impact a single developer can have.
I recently decided to package up some of that experience into a package and offer it for sale. I run https://goodgrids.com, which allows companies to automatically convert CSV files to nicely formatted Excel files. This project is fairly new, but it is very personally rewarding.