I've followed Todo.txt from the beginning. It's an interesting project because it started without code, as just a Lifehacker post (back when it was very popular) about GTD-esque formatting for text files [0]. The actual application was built later [1] to simplify creating/querying todos in the files.

[0]: https://lifehacker.com/geek-to-live-list-your-life-in-txt-16...

[1]: https://lifehacker.com/geek-to-live-reader-written-todo-txt-...