My template engine, SquirrellyJS. It has a syntax like Handlebars and Mustache, but is extremely fast: according to this benchmark, it's faster in 90% of tests than every other template engine.
I also made a documentation website for it, which took a lot of time but which I'm proud of as well.
It's been a project of over a year, but I can finally say that I have a working project! It's a nice feeling.