* Recursive file system management tools. Most developers cannot navigate a tree model and cower in great fear

* performance analysis/monitoring tools. Most developers are astoundingly bad at performance and then lie about it with weird unfounded guesses

* markdown to html parser or the opposite. Most developers have no idea the actual cost of involvement required for basic string parsing and wrongly believe it’s free

* network proxy. Aside from authentication this is mind blowingly simple. Most developers will supremely over engineer this

* API documentation. In a world where the average developer struggles to write an email actually describing inputs, purpose, and outputs clear enough for a stranger to follow can be quite the impossibility

A video game. Any kind of video game.

It is fun to develop, rewarding and the dynamics inside the code are always interesting (input from the player, performance, graphics, etc).

Even a simple game that can take an afternoon or a day to develop bring some fresh air.

I think writing a book about something technical. Either self-published (I like leanpub) or through a publisher. Doesn't have to be long, even a 30 page ebook is a big effort.

I have done this a few times and you learn so much and appreciate so much.

Other than that, I think it really depends on where you are starting. If you are a webdev, building an MVC framework. If you are a system programmer, building a basic HTTP.1/1 web server.

Doing a programming language/compiler was very educational and did a lot to demystify computers for me.

I would also second the recommendations to write a game. If for no other reason than the fact that it is some of the most fun programming there is, you usually get to flex muscles you'd never get to in a boring web dev job.

A ProjectContractChargingPeriodProjectAccountReferenceVMFactoryBuilderStrategy Implementation

Text editor, compiler, BASIC interpreter, virtual machine emulator, CRUD application, web server, Lisp interpreter, Forth Interpreter, SQL database application.

Repeat the above as gui applications, and on the web.

An ERP and a blog engine with the first post being about it

Automation tools for your own use. I did my own job offer web scraper. I've saved lots of time and pain automating work.

A video game engine

An overdesigned, overengineered personal website

JPEG codec

MPEG 1 codec

Minimal HTTP server of actually used features.

A raytracer and an emacs mode.

An ecommerce website.

Bots

A Todo list /s