Honest pick: the IoT protocols course from my exchange semester in Sweden. Three projects, all in Rust: an HTTP/REST server, a CoAP client, and an MQTT broker, written completely from scratch. No libraries. The course rule was you couldn't use anything, so you actually had to understand what TCP, headers, and message brokers really are instead of importing them.

That's the most technically interesting because it's the only time the abstractions were taken away. Everything else I build is React and TypeScript at a distance from the wire. Writing an MQTT broker by hand taught me more about how the web works than any framework ever has.

A close second is what I own at Capia: I turned a one-line brief, "build a chatbot", into a workspace-scoped analytics agent, plus an observability setup with OpenTelemetry across 8 repos into self-hosted SigNoz, so every trace from a frontend API call down to the DB query is visible. Less "look how hard this was", more "how do I know it actually works".

If you want the scrappiest story, there's also the Chrome password manager I built in 2022 with AES encryption before Firebase and local key storage. Three years before the AI boom, I was already prompting GPT-3 for a grammar checker. Timing beats skill sometimes.