* A database (transaction, lock managers, buffer/IO management, etc).

* An MVC web framework.

* A GUI validation library.

* A JavaScript UI library.

* An iteratee implementation.

* An Erlang-style actors library in another language.

* Implementing for-yield, async-await, etc on top of delimited continuations.

* Interpreters, typecheckers, code generators.

* Some of an ECMAScript implementation.

* Concurrent data structures: futures/promises, queues, etc.

* A simple roguelike game.