In the summer of 2006, just a month or so before going up to Cambridge for my freshman year, I took a couple of weeks out of my summer break to build Oxbridge Admissions. The site was inspired by an earlier, similar site that had existed for a few years, but had become defunct.
The goal was to help level the playing field for everyone applying to Oxford and Cambridge. For many years, elite boarding schools in the UK and abroad had collected information from their students who applied to the two universities. After coming back from interviews, each student would write a report about the experience. This meant that when a student at those schools decided to apply, they had access to hundreds of written profiles about what to expect and how to prepare. For a young person applying to the venerable universities shrouded in mystery, this was a significant advantage. Unfortunately, those applying from less selective schools, or from schools abroad who rarely if ever sent anyone to Oxbridge, did not have the same advantage. This seemed grossly unfair.
Long story cut short, sitting on the terrace of my parents' house in the Czech Republic, I hastily coded a basic version of the site, and recruited a few friends to help with moderating the submitted profiles. We also put in the work to find the profiles from the earlier site on archive.org and manually copied & pasted many of them into the new site. By August 2006, the primitive site was up and running.
Initially, we had some ambitions to add to and improve the site over time, but those never materialized. During the almost 13 years since the site launched, I probably spent a total of less than a day on updating the code or even maintaining the box it runs on. During that period, it probably took 2-3 hours per year to review and approve newly submitted profiles.
Despite this minuscule effort, the site still ranks #2 to #5 on Google for "Oxbridge Admissions". Over those 13 years, tens of thousands of people have read the profiles.  Given that every year, less than 20,000 people apply to either university, that's an amazing number of students who benefited from a more equal footing when applying.
To me, this is just yet another example of the power of product-market fit, even for non-profits: if you get product-market fit right, you can mess up (or not do at all) a lot of other things and still create a lot of value for your users.
 Of course, the site looks accordingly — like something that was thrown together quickly in 2006 and not touched since then. Given its ongoing popularity, the site does deserve an update though — I'm working on one that could last for the next 13 years.
[ 2] Dozens end up coming back afterwards to submit a profile of their own.