I built
https://nuenki.appIt's a browser extension that translates sentences that are at your knowledge level into the language your learning, thus immersing you in language learning as you browse.
It's my first foray into entrepreneurship, and I've learnt an incredible amount - both technically and in the soft skills of building the right thing and marketing it.
I'm also pretty proud of what I've built. I think it's genuinely useful, and I'm constantly upgrading it.
Earlier in the year I was still at school, and I built a novel firewall evasion system along with a report. I really ought to clean it up and publish it.
It's designed for the specific niche of corporate firewalls that only allow MITMd HTTP traffic, without even allowing websockets etc. I tested it against my school's firewall.