Email is one actually. Notoriously hard to work with from a systems point of view, and based on a protocol that's been minimally updated since it's conception in the early 90's. It's essentially been 25 years of the same old crappy standards that have been duck-taped to hell to try and fix security vulnerabilities. I always meant to re-write a version of email that works better with servers and API's. Such as JSON message formats, async key handling natively (no need for PGP anymore), sender identification and authoring, etc. But it's a massive undertaking, and I can't do it alone with my work schedule.