http://ohlife.com was a YC-backed [1] personal journal you maintained by replying to emails, which is super cool IMO.

Although I always thought maybe whatsapp would be a better fit for an interface, so I started building Diarist [2] for a Twilio hackathon. Never finished it though.

[1] https://news.ycombinator.com/item?id=1613137

[2] https://github.com/caruano95/diarist