Oh I am so familiar with this feeling. I recently found a program I wrote at my last job on a flash drive. It came from a chance conversation with one of the scientists I worked with.
After listening to her lament the many hours of manually parsing log files it took for her team to generate reports, I wrote a short program one day (with a few updates over the following weeks) to do it for them. Some days I feel that the code I wrote that one slow afternoon was the most useful thing I've done in terms of ROI. A couple hours of fun text manipulation and voila: a bunch of Very Happy People (tm) and untold hundreds of hours of time saved.